3x plus y equals 17

4x - y equals 18

3x+y=17

4x-y=18

4x-y=18

subtract 18 from both sides

4x-y-18=0

add y to both sides

4x-18=y

subsitute 4x-18 for y in first equation

3x + (4x-18) = 17

7x-18=17

add 18 to both sides

7x=35

divide both sides by 7

x=5

subsitute

y=4x-18

y=4 (5) - 18

y=20-18

y=2

x=5

y=2