A^2-12a+36

Factor. Check by expanding.

a2 + 6a + 6a + 36

a • (a+6)

6 • (a+6)

(a+6) • (a+6)

Multiply (a+6) by (a+6)

The rule says : To multiply exponential expressions which have the same base, add up their exponents.

The product is therefore, (a+6) (1+1) = (a+6) 2

Final result : - 1 • (a + 6) 2