If 6 is one root, then (x - 6) is a factor. So: (x - 6)(x + ?) = x2 - 2x + a
When you add ? to -6 you must get -2, so ? = 4
The factors are (x - 6)(x + 4) = x2 - 2x + 24
If 6 is a root , when you put in x= 6 the eq should = 0
62-2(6) +a = 0
36-12+a = 0
a = - 24