If f(x)=3x+1 then f(a+h)-f(a)
f(x) = 3x+1put x = a+hf(a+h) = 3(a+h) + 1f(a+h) = 3a + 3h + 1put x = af(a) = 3a + 1f(a+h)-f(a) = 3a + 3h + 1 -(3a+1)f(a+h)-f(a) = 3a +3h + 1 - 3a -1f(a+h)-f(a) = 3h