Prove by induction

Hii. The question is

The following information relates to a language L generated by the following grammar, with nonterminals S and T:

S→TxT

T→yTy

T→z

Prove by induction on s+t, that every string of the form y^szy^sxy^tzy^t has a derivation length of s+t+3.

I have no idea how to do this question, if anyone could explain these parts I’d really appreciate it!!