190 = x + 2(x+20)
190 = x + 2x + 40
190 = 3x + 40
190 - 40 = 150
150/3 = 50
x = 50
190 = 50 + 2(50 + 20)
one side of the width (the "short side", as it says) is the barn, so you don't count that because its only asking for the fencing. the length (which would be the long side, which implies that theres only one short side, if that confused you at all) is 20m longer. we figured out that 50m is the width so if both sides are 20m longer, the dimensions of the fencing would be:
50m + 70m + 70m