The length of a rectangular garden is three feet less than twice it’s width. If the perimeter of the garden is 42 feet, what is it’s length

13 feet

Step-by-step explanation: You are given that the perimeter, P, of the rectangle is 42 meters (i.e., P = 42). Also, the length, L, of the rectangle is 3 meters less than 2 times the width, W (i.e., L = 2W - 3).
Recall that the perimeter, P, of a rectangle is given by the following formula:
    P = 2W + 2L
Substituting 2W - 3 for L, we arrive at the following:
   P = 2W + 2(2W - 3)
   P = 2W + 2·2W + 2·-3  
   P = 2W + 4W - 6
   P = 6W - 6
Since we were given that  P = 42, then
    42 = 6W - 6
    42 + 6 = 6W - 6 + 6
    48 = 6W
    48/6 = 6W/6
    8 = W
Therefore, the width of the rectangle is 8 meters. Use this value to solve for the length:
    L = 2W - 3
    L = 2·8 - 3
    L = 16 - 3
    L = 13

Main idea on left, details on right