y = x^2 +2x
y = 3x +20
so:
x^2 + 2x = 3x + 20
subtract 3x from both side:
x^2 +2x -3x = 20
combine like terms:
x^2 -x = 20
subtract 20 from each side:
x^2 -x -20 = 0
factor the polynomial:
(x-5) (x+4) = 0
solve for x
x = 5 and x = -4 ( 5-5 = 0 and -4 +4 = 0 )
replace x = 5 in 2nd equation: y = 3(5) +20
y = 15 +20
y = 35
replace x with -4 in same equation:
y = 3(-4) +20
y = -12 +20
y = 8
so solution is (5,35) and (-4,8)