Calculate the length of the third side?

As you can see, this is a right triangle, meaning you have the Pythagorean Theorem at your disposal! 

This simply means that a² + b² = c², in which c is the hypotenuse and a and b are the adjacent sides. You have the hypotenuse, which is seven, and one of the adjacent sides, which is 6, so if you substitute them into the equation, you get:

6² + b² = 7²

Evaluate to get:

36 + b² = 49

If you subtract 36 from both sides, you get that b² = 13. So the answer is the square root of 13, which is approximately 3.61, or letter B.

Hope this helps!

C squared = a squared + b squared.

a = ? missing side
b = 6 leg
C = 7 longest side

7 squared = a squared + 6 squared

49 = a squared + 36
-36. -36

13 = a squared

Square root of 13 = a
So tge answer is b. 3.61