13. Simple problem when you divide a fraction by another you multiply by its inverse not by the same.
So 3x-12/x^2-9x+20 divided by 9x+54/x^2+x-30
Should be: 3x-12/x^2-9x+20 * x^2+x-30/9x+54
Now you can simplify it.
3(x-4)/(x-4)(x-5) * (x+6)(x-5)/9(x+6)
Or 3/(x-5) * (x-5)/9
Or 3/1 * 1/9
Or 3/9 = 1/3