In solving equations, whatever you do in the left side of the equation, you must also do the same thing with the right side to maintain equilibrium.
So that the left side will always be equal to the right side.
For example.
If we have x + 3 = 2x
If we subtract 3 from the left side, we must also subtract 3 from the right side.
x + 3 - 3 = 2x - 3
x = 2x - 3
That's the concept of the equality.