Hello!
To find the missing length, I believe you'll need to divide 31.5 by 6.3. To find the Area of a parallelogram, the formula would be A=bh. Once you've divided 31.5 by 6.3, you'll get 5 for your missing length. You can also check your answer by multiplying 6.3 w/ 5 to get 31.5
I hope this helps!