Inequality

x(x + 6) > 16 + 4x      expand L side

x^2 + 6x > 16 + 4x     subtract 4x from both sides of the inequality   and subtract 16 from both sides

x^2 + 2x - 16 > 0         Use Quadratic Formula to find       -1 - sqrt(17) >  x >  -1 + sqrt (17)