Sum and Difference Identity question

So far i have been able to do all the homework questions that i have for this, except these two.

Find the exact value for sin(x-y) under the given conditions.

sinx=24/25,0<x<pie/2; cosy=4/5,0<y<pie/2

and

sinx=2/3,pie/2<x<pie; siny=sqrt(7)/4,0<y<pie/2

Sry for writing out all the "pie's" and sqrt, but i don't know where to find the ascii equivalent.

Thanks!

P.S. Can you please explain if you skip steps or something because we just learned this and im trying to follow step by step.