5. 5x(x+2)
6. 3(x²-4x-2) only rational factorisation. You only need to spot the common factor 3. The rest is beyond your scope.
7. (x+5)(x+7) 5 and 7 are the only numbers whose sum is 12 and whose product is 35.
8. (x-9)(x+6) There are a number of factors of 54 but only 6 and 9 have a difference of 3.
9. (3x+4)(2x+7) Here you have various sets of factors, but trial and error gives you the ones you need.
10. (2x+3)(x-6)
11. (4x+5)(4x+5)=(4x+5)² is a perfect square (if you put x=0 into the original expression you get 25, which is a perfect square. If you put x=1 into it you get 81, which is a perfect square. So it looks like the expression is a perfect square for all values of x. Try x=-1 and you get 1, a perfect square. Now you can see that factorising it should be possible. You only need the first and last terms: 16x² and 25 with square roots 4x and 5 so factorising gives you 4x+5 when you combine them.)