Pythagorean Theorem states that C Squared = A squared + B squared
C = Hypotenuse
A = given side
B = x Squared
Plug numbers in:
2.9 squared = 2.6 squared + x squared
8.41 = 6.76 + x squared
8.41 - 6.76 = x squared
1.65 = x squared
x = Square root of 1.65
x = 1.28 (answer is rounded)