7. (x-7)(x-7)
8. (3x-5y)(3x-5y)
9. (x-15)(x+3)
10.(7m+6n)(7m+6n)
11. (2x+1)(2x+1)
12. (7x+2)(7x+2)
13. (p-18)(p+4)
Notice how 7,8,10,11, and 12 are all perfect squares. A good way to tell if a trinomial can be factored into a perfect square is if the square root of the coefficient of your variable multiplied by the square root of the constant (number with no variable) multiplied by 2 equals the middle term's coefficient.
For example, take 4x^2+16x+16. Taking the square root of 4 gives us 2. Taking the square root of 16 gives us 4. So, 2*2*4=16, which is our middle term, thus proving that this trinomial is indeed a perfect square.