Pre Calculus: How do you factor?

Question

Factoring questions on homework that I don't know how to do

7. ab+5a+3a+15

8. 4u(squared) - 3vw-3uv+4uv

10. 9x(squared)-49y(squared)

I can't really see how you would factor these

Answer 1

a(b+8)+15

4u² - 3vw + 1uv = 4u² + v(u-3w)

3(3x²-13y²)

Answer 2

I think that you have wrongly copied the first two as they don't properly factorise.

I suspect that #7 should be

ab + 5a + 3b + 15 = a(b + 5) + 3(b + 5) = (a + 3)(b + 5)

#10 factorises to (3x - 7y)(3x + 7y)

I don't know what you have done to #8.