[tex]f(x)=5x^2-5x-6
\\ \quad \\
\begin{cases}
f(\boxed{x+h})=5(\boxed{x+h})^2-5(\boxed{x+h})-6
\end{cases}\qquad thus
\\ \quad \\
\cfrac{f(x+h)-f(x)}{h}\qquad \textit{will be then}
\\ \quad \\
\cfrac{[5({x+h})^2-5({x+h})-6]\quad -\quad [5x^2-5x-6]}{h}
\\ \quad \\
\cfrac{[5(x^2+2xh+h^2)-5(x+h)-6]-[5x^2-5x-6]}{h}
[/tex]
[tex]\cfrac{\underline{5x^2}+10xh+5h^2\underline{-5x}-5h\underline{-6}\underline{-5x^2}\underline{+5x}\underline{+6}}{h}\impliedby \textit{canceling those ones}
\\ \quad \\
\cfrac{10xh+5h^2-5h}{h}\impliedby \textit{common factor}
\\ \quad \\
\cfrac{5\underline{h}(2x+h-1)}{\underline{h}}[/tex]
and surely, you'd know what that is
