compute f(c+h)-f(c) at the indicated point. f(x)=3x^2, c =1 so if c =1, then you know that f(c) = 3(c)^2 which would be 3(1)^2 which equals 3. therefore f(c)=3 however for f(c+h) i got 3(c+h)^2 = 3c^2+6h+3h^2 therefore f(c+h)-f(c)= 3c^2+6h+3h^2-3^2 ..im not sure if the answer should have that many variables in it.. Solution you forgot to put c=1 in your final expression f(c+h)-f(c)=3(c+h)^2-3c^2=3[c+h-c][c+h+c]=3h[2c+h] at c=1, thus f(c+h)-f(c)=3h[2c+h]=3h[2(1)+h]=3h[2+h]=6h+3h^2 now it has only one variable h.