To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $1+x$ and $4-x$ is $\left(x+1\right)\left(-x+4\right)$. Multiply $\frac{1-x}{1+x}$ times $\frac{-x+4}{-x+4}$. Multiply $\frac{1+x}{4-x}$ times $\frac{x+1}{x+1}$.
Since $\frac{\left(1-x\right)\left(-x+4\right)}{\left(x+1\right)\left(-x+4\right)}$ and $\frac{\left(1+x\right)\left(x+1\right)}{\left(x+1\right)\left(-x+4\right)}$ have the same denominator, subtract them by subtracting their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $\left(x+1\right)\left(-x+4\right)$ and $\left(x-1\right)\left(-x-1\right)$ is $\left(x-4\right)\left(x-1\right)\left(x+1\right)$. Multiply $\frac{3-7x}{\left(x+1\right)\left(-x+4\right)}$ times $\frac{-\left(x-1\right)}{-\left(x-1\right)}$. Multiply $\frac{4x}{\left(x-1\right)\left(-x-1\right)}$ times $\frac{-\left(x-4\right)}{-\left(x-4\right)}$.
Since $\frac{\left(3-7x\right)\left(-1\right)\left(x-1\right)}{\left(x-4\right)\left(x-1\right)\left(x+1\right)}$ and $\frac{4x\left(-1\right)\left(x-4\right)}{\left(x-4\right)\left(x-1\right)\left(x+1\right)}$ have the same denominator, subtract them by subtracting their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $1+x$ and $4-x$ is $\left(x+1\right)\left(-x+4\right)$. Multiply $\frac{1-x}{1+x}$ times $\frac{-x+4}{-x+4}$. Multiply $\frac{1+x}{4-x}$ times $\frac{x+1}{x+1}$.
Since $\frac{\left(1-x\right)\left(-x+4\right)}{\left(x+1\right)\left(-x+4\right)}$ and $\frac{\left(1+x\right)\left(x+1\right)}{\left(x+1\right)\left(-x+4\right)}$ have the same denominator, subtract them by subtracting their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $\left(x+1\right)\left(-x+4\right)$ and $\left(x-1\right)\left(-x-1\right)$ is $\left(x-4\right)\left(x-1\right)\left(x+1\right)$. Multiply $\frac{3-7x}{\left(x+1\right)\left(-x+4\right)}$ times $\frac{-\left(x-1\right)}{-\left(x-1\right)}$. Multiply $\frac{4x}{\left(x-1\right)\left(-x-1\right)}$ times $\frac{-\left(x-4\right)}{-\left(x-4\right)}$.
Since $\frac{\left(3-7x\right)\left(-1\right)\left(x-1\right)}{\left(x-4\right)\left(x-1\right)\left(x+1\right)}$ and $\frac{4x\left(-1\right)\left(x-4\right)}{\left(x-4\right)\left(x-1\right)\left(x+1\right)}$ have the same denominator, subtract them by subtracting their numerators.