中4...二次方程應用題!!

若每打酒的價錢增加$150,,以$900能買到的酒便少了12瓶..求一打酒的原本價錢
12瓶 = 1打
設一打酒的原本價錢為 $x
900 / (x + 150) = (900/x) - 1
900 / (x + 150) = ( 900 - x )/x
( x + 150 )( 900 - x ) = 900x
-x² + 750x + 135000 = 900x
-x² - 150x + 135000 = 0
x² + 150x - 135000 = 0
( x + 450 )( x - 300 ) = 0
x = 300 or x = -450 ( 捨去 )
Therefore, x = 300
一打酒的原本價錢是 $300