width = 3x + 1
As the length of the rectangle is twice the width then you need to multiply the width of the rectangle by 2, as follows:
2(3x + 1 ) = 6x + 2
To find an area of a rectangle you then need to multiply the width by the length:
Area = (3x + 1)(6x + 2)
= 18x² + 6x + 6x + 2
= (18x² + 12x + 2) in²
