x = 10
Here's how I got my answer:
Step 1: Add x to both sides, which leaves us with

Step 2: Simplify
to
, which leaves us with

Step 3: Add 21 to both sides, which leaves us with

Step 4: Simplify
to 20, which leaves is with

Step 5: Divide by sides by 2, leaving us with our answer, x = 10