Solve for 2 unknowns
3y + 3x =24
6y+3x = 39

The two unknown variable in the two given equations are "x" and "y". 
3y + 3x = 24
Dividing both sides by 3 we get
y + x = 8 
x = 8 - y
Putting the value of y in the second equation we get
6y + 3x = 39
Dividing both sides by 3 we get
2y + x = 13
2y + 8 - y = 13
y = 13 - 8
  = 5
Putting the value of y in the first equation we get
y + x = 8
5 + x = 8
x = 8 - 5
x = 3
So the value of x is 3 and the value of y is 5. I hope the procedure is clear enough for you to understand.