Question 156258
In this problem, you start by working with the variables you do know. xy +3x =25.  x=5. xy +3(5)=25, xy + 15=25. If 15 + xy=25, that means that 25-15=xy.  To follow the steps of the problem, from the beginning:
xy +3x=25, x=5.
5y +3(5)= 25
5y +15=25
5y +15 -15= 25-15
5y=10
5y/5 = 10/5
y=2

Check by substituting 2 for y:

5(2)+ 3(5)= 25
10+15=25
25=25