I kind-of need more specifics here. You didn't give any dimensions, but okay. So if you notice, from the side, any cone just looks like a regular old triangle. And we can treat it like one when trying to find the slant height. Ever heard of the Pythagorean theorem? a squared plus b squared = c squared. so in this case, the hypotenuse, or c-side, will be your slant height. take the square of the distance from the circle-base's center to the apex. add that to the square of the radius of the base. Find the square root (using your calculator) of that sum, and there's your answer.