Step One
Find the radius of the cone.
d = 6 which is given.
r = d/2
r = 6/2
r = 3
Remark
That's one of the sides of a right triangle. The other side is the slant of 9 inches. This is the hypotenuse.
Step Two
Find the height
h = ??
r = 3
Slant = 9
Use the Pythagorean theorem.
h^2 + r^2 = s^2
h^2 + 3^2 = 9^2
h^2 + 9 = 81 subtract 9 from both sides.
h^2 = 81 - 9
h^2 = 72
h = sqrt(72)
Step three
Express the answer
h = sqrt(72)
h = 8.49 <<<<<< Answer, but see below.
If you need this as a square root, then factor 72
72 = 6*6*2
h = sqrt(6* 6 * 2) The rule for square roots is if you have 2 of them you can take one outside the square root sign and throw the other one away.
h = 6 * sqrt(2) <<<< Answer