Explanation:
the highest exponent is 2, so, we expect to get 2 factors in x (as the multiplication of x×x = x²).
3 is a prime number, and the only factors to get 3 are 3 and 1
3 = 3×1
so, we expect to get factors that look like
(3x + a)(x + b)
when we do the multiplication, we get
3x² + 3bx + ax + ab = 3x² +(3b + a)x + ab
now we compare this to our original expression
3x² +(3b + a)x + ab = 3x² - 13x + 12
3b + a = -13
ab = 12
so, we know a and b must both be negative
(-×- = +, - - - = -).
the negative factors of 12 are
-12 × -1
-6 × -2
-4 × -3
which one of these pairs can make
3b + a = -13
be true ?
3×-12 + -1 = -37 no
3×-1 + -12 = -15 no
3×-6 + -2 = -20 no
3×-2 + -6 = -12 no
3×-4 + -3 = -15 no
3×-3 + -4 = -13 yes
so, our factors are
(3x - 4)(x - 3)