The
lateral area of a cone is the surface area (aka area of the sides of the object) minus the area of the base of the cone.
The equation for the lateral area of a cone is:

where r = radius and l = slant height.
Since you know that the lateral area =

and the radius, r = 50 cm, you can plug those values into the equation for the lateral area and solve for l, the slant height:
The slant height of the cone is A) 10cm.