3y^2-16y-12 factor by grouping

(y - 6)(3y + 2)

Take the y^2 coefficient 3 and multiply the constant by it, to get y^2 - 16y - 36

Find two numbers that multiply to 36 and add to -16, which are -18 and 2. Use these numbers as the second term in the polynomial, to get (y - 18)(y + 2).

Divide the numbers by the original coefficient
3, and if there’s a fraction, move the denominator to the coefficient of the variable.

(y - 18/3)(y + 2/3) —> (y - 6)(3y + 2)