>>780
まず、数式の正しい書き方、かっこの多用を心がけてね

Y = -(a^2/12) + a - 3
 = -(1/12) ( a^2 - 12 a ) - 3
 = -(1/12) { ( a - 6 )^2 - 36 } - 3
 = -(1/12) ( a - 6 )^2 + 3 - 3
 = -(1/12) ( a - 6 )^2