Question 60365
If {{{f(x)= x^2 + 5}}}
{{{f(a)=a^2 + 5}}}
{{{f(a + h) =(a + h)^2 + 5 = a^2 + 2ah +h^2 + 5}}}
{{{f(a + h) - f(a)}}} 
= {{{a^2 + 2ah +h^2 + 5 - (a^2 + 5)}}} Need to multiply the minus sign by each term in the bracket)
= {{{a^2 + 2ah +h^2 + 5 - a^2 - 5}}}
= {{{2ah +h^2}}} 

Note If this part of introductory calculus and the h is very, very small then h^2 would be insignificant and f(a + h) - f(a) would approximate to 2ah.)