if y =8' and z=the square root of 73 what is the length of x

Assuming z is the hypotenuse of a right triangle and not the 3rd dimension,

z = sqrt(73) = sqrt(x^2 + 8^2)

x^2 + 8^2 = 73

x^2 = 73 - 64 = 9

|x| = 3 is the length of x.

If x has direction it could be ±3.