Can somebody help me. I got the answer as 16x it says it’s wrong. Please help due at 11:59 tonight

Answer:
12x²+4x
Step-by-step explanation:
the formula for area of a rectangle is length * width so
area = 4x *(3x+1)
12x²+4x
When you multiply x by itself you get x squared and because 4x is not squared the cannot be combined.