You can see the g of x right over there. So you replace x with g of x for where in this expression, you get h of g of x, and that is capital F of x. Now why am I doing all of that? Well, this might start making you think about the chain rule, because if this is true, then that means that capital F prime of x is going to be equal to h prime of g of x, h prime of g of x, times g prime of x. And so what would that be? Well, we already know what h prime of x is. So, let me do this in another color.