$$(7a+b)^{2}-2(7ab)(7a-b)+(7a-b)^{2}; (a-b)^{2}=a^{2}-2ab+b^{2}$$
$a=\frac{-\sqrt{b^{4}+4b^{3}-4b^{2}-2bc+2c}+b^{2}}{14\left(b-1\right)}\text{, }b\neq 1\text{, }c\in \mathrm{C}$
$a=\frac{\sqrt{b^{4}+4b^{3}-4b^{2}-2bc+2c}+b^{2}}{14\left(b-1\right)}\text{, }b\neq 1\text{, }c\in \mathrm{C}$
$a=\frac{c-2}{14}\text{, }b=1\text{, }c\in \mathrm{C}$
$\left\{\begin{matrix}\\a=\frac{c-2}{14}\text{, }b=1\text{, }c\in \mathrm{R}\text{, }&\text{unconditionally}\\a=\frac{-\sqrt{b^{4}+4b^{3}-4b^{2}-2bc+2c}+b^{2}}{14\left(b-1\right)}\text{, }b\in \mathrm{R}\text{, }c\in \mathrm{R}\text{; }a=\frac{\sqrt{b^{4}+4b^{3}-4b^{2}-2bc+2c}+b^{2}}{14\left(b-1\right)}\text{, }b\in \mathrm{R}\text{, }c\in \mathrm{R}\text{, }&\left(b<1\text{ or }c\leq -\frac{b^{4}+4b^{3}-4b^{2}}{2-2b}\right)\text{ and }\left(c\leq \text{Indeterminate}\text{ or }b\neq 1\right)\text{ and }\left(b>1\text{ or }\left(b\neq 1\text{ and }c\geq -\frac{b^{4}+4b^{3}-4b^{2}}{2-2b}\right)\right)\end{matrix}\right.$
