We know that one factor is (x-3), so let's find the other factor.
(x-3)(x+a) = x^2 + 2x - 15
Expanding this, we get:
x^2 + ax - 3x - 3a = x^2 + 2x - 15
Now, we can compare the coefficients of the powers of x:
a - 3 = 2 (coefficient of x)
-3a = -15 (constant
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