Question 1021463
Your f(x+h) is incorrect.
{{{5(x+h)^2+7=5(x^2+2xh+h^2)+7=5x^2+10xh+5h^2+7}}}
Now continue,
{{{f(x+h)-f(x)=5x^2+10xh+5h^2+7-5x^2-7}}}