X/3Y=2XY/(6Y^2)，

3X/(2Y^2)=9X/(6Y^2)，

2. Common ground: 3a^2b^2

6c/(a^2b)= 18bc/(3a^2b^2)

c/(3ab^2)=ac/(3a^2b^2)

3. Common denominator: 2 (x+y) 2,

(x-y)/(2x+2y)=(x+y)(x-y)/[2(x+y)^2)

xy/(x+y)^2=2xy/[2(x+y)^2].

4. Common denominator: (2m+3)(2m-3),

2mn/(4m^2-9)=2mn/[(2m+3)(2m-3)]

(2m-3)/(2m+3)=(2m-3)^2/[(2m+3)(2m-3)]