What are the factors of expression (y-3)(x+7)

That is easy!! :)
y=3 and x=-7

When you find the factors of something like this, you have to just get the letter alone.

For example: y-3. You have to first write it as y-3=0, then you add 3 from BOTH sides of the = so that you can make the -3 go away and y will be alone. So, it will look like this: y-3+3=0+3. Then, you solve it for y (which just means that you want y by itself on one side of the = ). So, -3+3=0, which means they disappear from that side; since 0+3=3, that's what that side turns into, so what you end up with is y=3

Same thing goes for the x+7 equation:
x+7=0
x+7-7=0-7
x=-7