triangle b is right
using the converse theorem of pythagoras.
if the square of the longest side in a triangle equals the sum of the squares of the other 2 sides then the triangle is right.
longest side squared = 36² = 1296
26² + 18² = 676 + 324 = 1000 ≠ 1296
hence triangle a is not right
longest side squared = 25² = 625
20² + 15² = 400 + 225 = 625 ← correct
hence triangle b is right
in order to make a triangle with non-zero area, the sum of the shorter two sides must be greater than the longest side.
here, 4 + 4 > 7, so the lengths can form a triangle.