Cancel out $a+3$ in both numerator and denominator.
$$\frac{1}{a+1}+\frac{3}{a+1}-\frac{2a}{a+3}$$
Since $\frac{1}{a+1}$ and $\frac{3}{a+1}$ have the same denominator, add them by adding their numerators. Add $1$ and $3$ to get $4$.
$$\frac{4}{a+1}-\frac{2a}{a+3}$$
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $a+1$ and $a+3$ is $\left(a+1\right)\left(a+3\right)$. Multiply $\frac{4}{a+1}$ times $\frac{a+3}{a+3}$. Multiply $\frac{2a}{a+3}$ times $\frac{a+1}{a+1}$.
Since $\frac{4\left(a+3\right)}{\left(a+1\right)\left(a+3\right)}$ and $\frac{2a\left(a+1\right)}{\left(a+1\right)\left(a+3\right)}$ have the same denominator, subtract them by subtracting their numerators.
Cancel out $a+3$ in both numerator and denominator.
$$\frac{1}{a+1}+\frac{3}{a+1}-\frac{2a}{a+3}$$
Since $\frac{1}{a+1}$ and $\frac{3}{a+1}$ have the same denominator, add them by adding their numerators. Add $1$ and $3$ to get $4$.
$$\frac{4}{a+1}-\frac{2a}{a+3}$$
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $a+1$ and $a+3$ is $\left(a+1\right)\left(a+3\right)$. Multiply $\frac{4}{a+1}$ times $\frac{a+3}{a+3}$. Multiply $\frac{2a}{a+3}$ times $\frac{a+1}{a+1}$.
Since $\frac{4\left(a+3\right)}{\left(a+1\right)\left(a+3\right)}$ and $\frac{2a\left(a+1\right)}{\left(a+1\right)\left(a+3\right)}$ have the same denominator, subtract them by subtracting their numerators.
