This binomial expression can be solved bya process know as FOIL.
First- multiply the first terms of each expression togther; you get 3x(3x) = 9x2
Outside-multiply the outside terms of each expression together 3x(-6) = -18x
Inside-multiply the inside terms of each expression together 6(3x) = 18x
Multiply the last terms of each expression 6(-6) = -36
Now we put this all together in one expression as we get 9x2 - 18x + 18x - 36
Finally combine like terms and we get 9x2 - 36
Selecting the product for this expression is also based on the distributive property of multiplication where every term in the first expression is multiplied by every term in the second expression. If you do this you will get the same answer.
I hope this helps.