10x+15yz

5 goes into 10 2 times, and into 15 3 times, so factor it in front of a parentheses

5(2x+3yz)

------------------------
3y(4-y)+6x(4-y)

Take out the red factor in front just like we took 5 out of
the first problem:

(4-y)(3y+6x)

Then 3 goes into 3 1 time and 3 goes into 6 2 times so now in the
end we have:

(4-y)3(y+2x)

But it's customary to put the shortest factor first, so move 
the 3 out front:

3(4-y)(y+2x)

--------------------------------

2a-6-2a^2+3b^2

That doesn't factor.

Edwin