we have x^4-13x^2+ 36 \le 0
or (x^2-9)(x^2-4) \le 0
or 4\le x^2\le 9
so we get 2 ranges for x
1) -3\le x\le -2
2) 2\le x\le 3
we need to find the maximum of x^3-3x in this ranges
x^3-3x = x(x^2-3)
in the 1st range x is negative and x^2-3 is positive so in the range x^3-3x is -ve
for x\gt \sqrt{3} and hence for x \ge 2 both x and x^2- 3 are positive and increasing so the highest value is at highest x that is x = 3 so the value of x^3-3x=18
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