-x=51-20y
times -2 both sides
2x=102+40y
since 2x=-78-20y
-78-20y=2x=102+40y
-78-20y=102+40y
add 20y both sides
-78=102+60y
minus 102 both sides
-180=60y
divide both sides by 60
-3=y
sub back
-x=-51-20y
-x=-51-20(-3)
-x=-51+60
-x=9
x=-9
(x,y)
(-9,-3)
