x^3 - 21x = -20
add 20 to both sides
x^3 - 21 x + 20 = 0
Rational Root Theorem:
Factors of P (constant) 20 = 1, 2, 4, 5, 10, 20
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Factors of Q (leading Coefficient) = 1
Possible zeros (all + -) 1/1, 2/1, 4/1, 5/1, 10/1, 20/1
Of the choices given only the number 1 is a possible root.
