2x^2 + 17x -21 cannot be factored nicely, if it was +21 you could, but the zeros for this have to gotten using the quadratic equation
-b +/- (b^2-4ac)^1/2
2a
so
-17 +/- (17^2-4(2)(-21))^1/2
2(2)
-17 +/- (457)^1/2
4
so the zeros are roughly 1.094 and -9.594
