I'm guessing you want to find what ? is.
Okay. The perimeter is the sum of all of the sides, right, so we can set up an equation. Since I'm too lazy to have to hit the caps button thingy each time, the ? mark will just be represented by a letter, y.
y+x+x+3x+1=6x+2
Simplify the left side of that.
5x+1+y=6x+2
We want to find what y (or ?) is, right? So we can just solve for y in the equation.
y=(6x+2)-(5x+1)=x+1
The ? stands for x+1.
Hope this helps!