Hi,

First of all, x + -3y is equal to x - 3y.

If x - 3y is 7 and -x + 4y is also 7 then

(x - 3y) + (-x + 4y) = 7 + 7 = 14.

Simplify

(x - 3y) + (-x + 4y) = 7 + 7 = 14

and solve for y. Once you know the value of y you can substitute it into one of the two initial equations and then solve for x.

The reason this works is that the first equation contains x and the second contains -x so that when you add the left sides and simplify the x term is eliminated. This method of elimination is sometimes useful in solving systems of equations but you may need to manipulate the equations first before you add them so that the addition eliminates one of the variable.

I hope this helps,
Penny