Each side of the square picture frame is t centimeters long. The perimeter is t+t+t+t = 4*t centimeters long, which is just the entire distance around the frame. If we had the perfect amount of ribbon, then d = 4*t would be the end of the problem; however, we're 10 cm too short meaning that d is too small and it's actually d = 4*t - 10
For example, let's say that t = 12 meaning that the perimeter is 4*t = 4*12 = 48 cm. This would mean we have d = 4*t-10 = 4*12-10 = 48-10 = 38 cm of ribbon, and we're 10 cm too short (comparing 38 to 48).
