20. isosceles means two sides are the same length, so you take the ten off the perimeter to get 26 remaining cm (36 -10) and you then divide that by two to get both sides of 13 cm (26 / 2)
21. because you know that if you split the triangle down the middle you get a right triangle and you can use a^2+b^2=c^2 to find the height, so we take the one side of 13 cm and half of the third side, which is 5, and you can find your answer (a^2 + 5^2 = 13^2 ---> a^2 + 25 = 169 ---> a^2 = 144 ---> a = 12)
