To solve this problem, we need to know that perimeter is the combined length of the sides. A triangle has three sides. Side 1 + Side 2 + Side 3 = Perimeter. In other words, (3y+z) + (4x-y+z) + something = (5x-2y+3z). First, subtract (3y+z) from (5x-2y+3z). Then subtract (4x-y+z) from that answer. This will give you the length of the third side. Hopefully this gives you a good starting point! If you're still unsure, feel free to let me know.
