Hello arzeIt's difficult to define any hard-and-fast rules. Obviously, you have to eliminate between the two equations, so you'll have to look for ways to do this. You could try to isolate and then eliminate it by substitution.

In this case, the equations only differ in the power of in the denominators on the right-hand-sides. So

Now make the subject of this equation, and eliminate it by substituting into whichever of the original equations looks like being the easier.

Grandad