From the top (12") side to the bottom (9") side is 11" (found by adding the two vertical values (7" and 4") on the left-hand side.
Since there is a 3" segment on the top-right, the bottom-right length is 8" (11" - 3").
The horizontal value at the bottom-left is 8"; the horizontal value at the bottom-right is 9", for a total of 17".
Since the top-left length is 12", the top-right length is 5" (17" - 12").
Now, draw in some line segments and see if you can finish this.
Good luck!