Hello.
To do this, work from right to left, or inside out. (Same thing).
First, take u(2).
u(2)= 6(2)-7
= 12-7
= 5
Now plug 5 into s(t).
s(u(2))= (5)² - 4
= 25-4
= 21
** Answer: (proper notation)
s(u(2))= 21
Hope that helps!
***************************
Disclaimer:
Always double check your answer with a reliable source, as mistakes can be made.