Explanation:
Hello!
For this you multiply the numbers that intersect at that box
Box A 2x and 4 intersect
2x * 4 = 8x
Box B 3 and 4 intersect
3 * 4 = 12
Box C 2x and x intersect
2x * x =

Box D 3 and x intersect
3 * x = 3x
To solve this we put all these together

Combine like terms

Hope this helps!