1. x/y=a/b ; solve for Y.

Does anyone know how to solve this? I believe you gotta cross multiply but I'm lost from there. any help would be great. Thank you!

cross-multiplying is just a shortcut. Using the normal method of clearing fractions, the steps are

x/y = a/b
x = ay/b
bx = ay
bx/a = y

or, using the shortcut of multiplying everything by by, you have

byx/y = aby/b
bx = ay
bx/a = y