The equation to use to find the perimeter of a trapezoid is P = a + b + c + d. In this equation the P stands for perimeter, while the a, b, c and d each stand for one side of the trapezoid. In order to use this equation, you need to know the values of each side of the trapezoid.
To fill in the values for the equation, look at each side of the trapezoid. The two sides that are parallel to each other are the bases, and are the a and b sides. The remaining two sides of the trapezoid are the c and d sides. Take the value for each of these sides, and fill it into the equation.
Add up the values of the equation to get the perimeter of the trapezoid. For example, if the equation is P = 4 + 6 + 2 + 3, the answer would be P = 15.