Value of x in the solution to the system of equations.
6x+3y=13
3x-y=4

The two easiest ways to solve a system of equations (in my opinion) are to add the equations together while getting rid of a variable, or solving for a variable in one equation and plugging it into another. The latter is the method I will use.

3x - y = 4 (solve for y)
3x = 4 + y
3x - 4 = y
y = 3x - 4 

Now we can plug y into the other equation:
6x + 3y = 13
6x + 3(3x - 4) = 13
6x + 9x - 12 = 13
15x = 25
x = 25/15
x = 5/3
The answer is x = 5/3

If you ever needed to find y as well, you can plug this into either equation to find y.
3x - y = 4
3(5/3) - y = 4
5 - y = 4
y = 1