Answer:
1). This is a right angle triangle = TRUE
2). The hypotenuse of the triangle has a length of 21 = FALSE ("C" is the hypotenuse which makes 21 and 20 the leg of the triangle)
3). One of the legs has a length of 20 = TRUE
4). The length of "c" can be found by 202 + c2 = 212 = FALSE (To calculate the hypotenuse of the triangle you should use the formula a² + b² = c² )