Set up a proportion.
24/x=9/15
24(15)=9x
360=9x
40=x
x=40
This is because if side one of the smaller triangle is 9 and side two of the smaller triangle is 15, and if side one of the larger triangle is 24 then side two of the larger triangle has to be 40 because they are proportional.
I hope this helps!
Best wishes :)
