Answer
$12x+4$
Work Step by Step
The perimeter of a square is equal to the sum of the length of each of its four sides. Also, we know that a square has four sides of equal length.
Length of each side: 3x+1
$(3x+1)+(3x+1)+(3x+1)+(3x+1)=4\times(3x+1)=12x+4$