You cannot change two existing, completed equations so that they are equal to each other. However, when working with two equations, you may set them equal to each other to solve a system of equations. An example is the system of
2x+5y=10
3x-5p=10
You may now combine the two, as they both are equal to ten. This results in the eqation of
2x+5y=3x-5p
You may simplify this to
5y=x-5p
This brings you one step closer to solving, and one may complete the system with some additional information.