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