The area of a cone formula is
but you would use that if you were given the height of the cone, not the length of the slant side of the cone as in your problem. So you need to find the height of your cone.
To determine the height of your cone:
Since you are given the length of the slant side, you need to find the height of the cone itself. A cone is made from a right triangle that is rotate around itself, right? You are given one of the legs of the triangle, it is the radius of your cone. You are also given the length of the hypotenuse (the slant side). Use the Pathagorean theorum
to find the height of the triangle which is also the height of your cone. Then you can use the formula for the area of a cone to find your answer.
Remember that a & b of your Pathagorean equation are the legs, and c is your hypotenuse.
Does this make sense? Good luck!