Complete the square: x^2 - 10x + 21 + 3x^2 - 2x + 8.
\(x^2-10x+21+3x^2-2x+8 \) can be simplified, first, to \(4x^2-12x+29\). We know 4x^2=(2x)^2 and -12x=2x(-6), so we have \(\boxed{(2x-6)^2-7}.\)
