It depends on what information you do have. That is not made clear in the question.
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Why do you need to FIND the slant height if you have the [lateral height and] slant height?
Slant height is 39.98 cm
Assuming it is a right cone, use Pythagoras - slant height = hypotenuse, other two sides = radius of base, and height.
By using trigonometry or Pythagoras' theorem
Uisng the lateral area and tha radius, you should be able to find the height of the cone. Using the height and radius as the legs of a right triangle, use the Pythagorean Theorem. The hypotenuse is the slant height.