Let W = the width of the frame and L = its length. You have two unknowns, W and L, so you need two equations to solve it.
Equation 1 - The length is 20 cm more than triple the width; that is L = 20 + 3W
Equation 2 - The perimeter of the frame is 112 cm. The perimeter of a rectangle is 2L + 2W, the distance around the rectangle. So
112 = 2L + 2W
Now substitute equation 1 for L in equation 2:
112 = 2(20 + 3W) + 2W
This is the key step, where you substitute to get rid of one of the unknowns, leaving a solvable equation in only one unknown (W in this case):
112 = 40 + 8W
Solve for W then use L = 20 + 3W to get L.
To check your answer, add 2W and 2L and it should add up to 112.