Answer:
2√6
Explanation:
find the hypotenuse of the triangle thats 3 and 7
pythagorean
3 squared + 7 squared
16 + 49 = 57
√57 is the hypotenuse
√57 is also the base of the triangle that has side thats 9
pythagorean theorem
x squared + √57 squared = 9 squared
x^2 + 57 = 81
x^2 = 81 - 57
x^2 = 24
x = √24
x = √4 * √6
x = 2√6