Use the Pythagorean Theorem: a² + b² = c².
a and b are the leg lengths and c is the hypotenuse.
The hypotenuse is the longest side of the three, so you know the hypotenuse is 7 cm.
We'll make 2 cm = a, and the third side = b.
Plug these into the formula.
2² + b² = 7²
4 + b² = 49
b² = 45
Square root b and 45 to find the length of the third side.
√b = √45
b = 6.71
The third side is 6.71 centimeters.