+128407 x(3x - 4) ≤ 20
3x^2 - 4x - 20 ≤ 0
(3x - 10) ( x + 2) ≤ 0 (1)
Set factors to 0 and solve for x to find the critical points
x = 10/ 3 x = -2
We have three intervals that we can test
(-inf, -2 ] , [ -2, 10/3 ] and [ 10/3, inf )
Picking an x in each interval to test in (1) shows that the solution = [ -2, 10/3 ]
