Hello,

You have

What does FTC part 1 says ?

So now, here is what I propose to you : arrange the original integral in order to get a smilar form to what the FTC gives.

(because in the FTC, t is on the upper boundary - and there's a - sign because you inverted the order of boundaries)

Now, what if you let ? You will be able to let t in the upper boundary of the integral. And

Hence

Apply FTC :

Apply chain rule in the LHS :

Substitute back :

Does this look clear enough ?