What is the missing number in the synthetic division problem below?
(everything in picture)

The answer is C, 10.

When performing synthetic division, we bring down the first number in the box.  We then multiply this number by the number outside the box, and place that result under the second number in the box.  We add these two numbers and bring that sum down.

We then continue this process by multiplying this second number by the number outside the box, and place the result under the third number in the box.  We then add the two numbers in the third column and bring the sum down.  The missing number is under this column; that means it is found by adding 2+8, so it is 10.