Explanation:
if the solution has "nice" numbers, we can split the expression into 2 factors in the form
(x + a)(x + b) = x² + (a+b)x + ab
as this is equal to x² + 4x - 21.
ab = -21
so, one is positive and one is negative.
and 4 = a + b
we know the typical factors of 21 : 3×7
and then we see easily : 7-3 = 4
so,
we have
(x + 7)(x - 3) >= 0
this has 2 x-intercepts (x-values when y = 0) :
x = -7
x = 3
because the coefficient of x² is positive (1), we know that the parabola (or quadratic equation) is opening upwards.
that means the y-values for x-values between -7 and +3 are negative (< 0), but the y-values for everything else are positive.
so, the solution are the intervals
x <= -7 and x >= 3
in these areas is the expression x² + 4x - 21 >= 0.