Start by moving the x over to the right side by subtraction to start isolating y:
-3y = -5x + 15
Now divide completely isolate y by dividing -3 from both sides:
y = (-5/-3)x + (15/-3)
Simplify:
y = 5/3 x + (-5)
This becomes
y = 5/3 x - 5
or
y = 5x/3 - 5