The diagram gives rise to two equations in the two unknowns.
3x = 90
2x +3y = 90
The first equation tells you
x = 90/3 = 30
Using that value in the second equation gives
2*30 +3y = 90
3y = 30 . . . . . . . . . subtract 60
y = 30/3 = 10 . . . .. divide by 3
The value of y is ...
y = 10
