What do you do before adding the equation so that one variable will be eliminated when you dd them 2x-4y=5 and 6x-3y=10

We have a system of equations 2x - 4y = 5 and 6x - 3y = 10.
We want to eliminate one variable when we add up the equations.

If we want to eliminate the x variable, we need to multiply the top, bottom or both of the equations with a number that when the equations are added together will eliminate the x variable.

Multiply the top equation by -3.

-3(2x - 4y) = 5 * -3
-6x + 12y = -15

Now when we add the two equations together, the x's will be eliminated.

-6x + 12y = -15
6x + 6y = 10
18y = -5

The x's were eliminated.