First you must solve system of equations.
Multiply the first equation by 4 and second one by 3.
You result with,
![16x-12y=4 \\15x+12y=27](/tpl/images/0074/0364/03c4d.png)
Add the equations so y terms cancel out.
![31x=31\Longrightarrow x=1](/tpl/images/0074/0364/ad91e.png)
Insert x that was found in either one of the equations. I'll pick first one.
![4\cdot1-3y=1](/tpl/images/0074/0364/c7649.png)
Solve for y.
![-3y=-3 \\y=1](/tpl/images/0074/0364/9364c.png)
The solutions to the system of equations are,
![\boxed{x=1},\boxed{y=1}](/tpl/images/0074/0364/81e9e.png)
Therefore the number missing is 1.
Hope this helps.
r3t40