Answer:
A = 6x² + 5x + 1
Explanation:
The area of a rhombus is
where d₁ is one diagonal and d₂ is the other diagonal.
If we have the rhombus ABCD, the diagonals are AC and BD.
The problem tells us that AC = 3x + 1 and BD = 4x + 2
Substituting this in our formula for the area and simplifying as much as we can (by factorizing) we get:

Therefore the area is A = (3x + 1) (2x + 1). If you want to solve the parenthesis you'd get A = 6x² + 5x + 1