You would need to find factors of -180 that sum to 3. Those would be 15 and -12, so the factorization is
... (p -12a)(p +15a)
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If we assume the factors are
... (p + m·a)(p +n·a)
then their product is
... p² + (m+n)·pa + m·n·a²
Comparing this to the given trinomial, we see m and n are factors of -180, and their sum is 3.
