what we can do is set up an algebraic equation.
our variable can me L for Length
3L+4L+5L=90
now all we have to do is solve
12L=90
---- ----
12 12
L=7.5
now multiple each number in the ratio by 7.5 to find out how long each side is
3 times 7.5 is 22.5
4 times 7.5 is 30
5 times 7.5 is 37.5
the side lengths are 22.5, 30, and 37.5