That answer is correct; better shown as: (a^2 - c^2) / (2b)

Your expression is somewhat ambiguous (even if correctly shown); this is clearer:

(a/b - c/b) * [a - b(a/b - c/b) / 2]

Try this way:

x = (a/b - c/b) ; so expression becomes:

x * [(a - bx) / 2]

Continue...then substitute back in...OK?