4x^2 - 10x - 84 factor out the 2
2(2x^2 - 5x - 42) = 2(2x + 7)(x - 6)
If you are looking for the roots set it equal to 0
2(2x+7)(x-6) = 0
2x+7 = 0 or x - 6 = 0
2x = -7 or x = 6
x = -7/2
CHECK:
2(2x+7)(x-6) = 0
substitute in -7/2 for x
2(2*(-7/2) + 7)((-7/2) - 6) = 0
2(0)(-19/2) = 0
0 = 0 check it works
2(2x+7)(x-6) = 0
substitute in 6 for x
2((2*6)+7)(6-6) = 0
2(12-7)(0) = 0
0=0 Check it works
x = (-7/2,6)