Can you form a right triangle with the three lengths give 20,21,29

N a right triangle, the hypotenuse is opposite the largest angle in the triangle (90 degrees). So, the longest length would be the hypotenuse. 
You can use the Pythagorean theorem to check if the given lengths are a right triangle: 
a^2 + b^2 = c^2, where a and b are the lengths of the legs, and c is the length of the hypotenuse. 
If the equation turns out to be true and both sides are equal, the triangle is a right triangle. 

20^2 + 21^2 = 29^2 
400 + 441 = 841 
841 = 841 

Because both sides of the equation are equal, the triangle is a right triangle.