3x^2 + 3x - 6
First factor out the 3 in front of x^2:
3( x^2 + x - 2 )
Now we can look at the inside of the brackets, and find two numbers that, when multiplied, give -2, and when added, give +1. You will usually find them if you create a list of factors of the constant term (-2 in this case).
2 * -1 = -2
2 + -1 = 1
So the factorization becomes 3(x+2)(x-1)