1) 7k/18p-4k/18p=7k-4k/18p=3k/18p=k/6p
2) a-b/2b-a/2b=a-b-a/2b=-b/2b=-1/2
3) - a-12b/27a + a+15b/27a=a+15b/27a - a-12b/27b=a+15b-a+12b/27a=27b/27a=b/a
4) x-7y/xy - x-4y/xy=x-7y-x+4y/xy=-3y/xy=-3/x
5) 10a+6b/11a^3 - 6b-a/11a^3=10a+6b-6b+a/11a^3=11a/11a^3=1/11a^2
6) x^2-xy/x^2y +2xy-3x^2/x^2y=x^2-xy+2xy-3x^2/x^2y=xy-2x^2/x^2y=x(y-2x)/x^2y=y-2x/xy