Simply each equation is manipulated by the x in the way that the function is telling you to
x = a
f (a) = 8 + 2(a)^2
x = a + h
f (a + h) = 8 + 2( a + h) ^ 2
the last case is represented by two scenarios
x = a + h ... x = a/h
f (a + h) - f( a / h) =
(8 + 2(a + h) ^2) - (8 + 2(a / h)^2)