(2x + a)(x +3) = 2x^2 + 4 ax + b
Expand the left side
2x^2 + ax + 6x + 3a = 2x^2 + 4ax + b subtract 2x^2 from both sides
ax + 6x + 3a = 4ax + b simplify
(a + 6)x + 3a = (4a)x + b
This implies that
a + 6 = 4a → 6 = 3a → a = 2
And......
3a = b
3(2) = b = 6
