solve the systems of equation:

3x+y=20    ⇒  y =  20 - 3x   (1)

2x+3y=18   (2)

Sub (1)  into (2)  for y

2x + 3 [ 20 - 3x]  =  18    simplify

2x + 60 - 9x  =  18

-7x + 60  = 18       subtract 60 drom both sides

-7x  =  -42            divide both sides by -7

x = 6

And using (1)    y =  20 - 3(6)  =  20 - 18  =  2