We need to find two numbers that multiply to -54 and add to +15.
This means that one number will be negative and one will be positive. (a negative times a positive is negative)
Also the larger of the two numbers will be positive, since when added it results in a positive number.
Let us list the factors of -54 that fit this criteria.
-1*54 = -54.
-2*27 = -54
-3 * 18 = -54. and hey! 18 + -3 = 15
So our answer is (x - 3)(x + 18)
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