Hi there!
Your question is mistaken. You must have meant to ask for the value of x.
By definition of a parallelogram, we know that there must be two pairs of parallel sides. We also know that a parallelogram will follow this form:
 See attachment.
Therefore, we know that 
 . Let's solve:
. Let's solve:

 Subtract x from both sides.
 Subtract x from both sides.
 Add 15 to both sides. Now the variable is isolated.
 Add 15 to both sides. Now the variable is isolated.
 Divide both sides by 5.
 Divide both sides by 5.
So as you can see, x = 6. If you tried to solve this and got a negative solution, remember that lengths and distances can never be negative (e.g. I can't go -5 miles; a square can't have a length of -3 feet, etc.).
Hope this helps! :^)