Question

if f(x)=x^2+3x+5, what is f(a+h)

Answer 1

Put (a+h) where x is and simplify to the extent you desire.
  f(a+h) = (a+h)^2 +3(a+h) +5
  = a^2 +2ah +h^2 +3a +3h + 5
  f(a+h) = a^2 +3a +5 +h(2a +3) +h^2