A classic right triangle is the 3, 4, 5 triangle. Essentially, 3^2 + 4^2 = 5^2 . Otherwise known as, 9 +16 = 25. With that said, there are two of those that we can make by drawing horizontal lines along the grid. We also know from counting the two vertical sides are each 4 units. To summarize, two sides are 5 units and two are 4 units. We add them together, 5 + 5 + 4 + 4 = 18
