How can i find the surface area of a right cone with a radius of 6 and slant height of 16

First, you have to calculate the height.
Pythagorean Theorem:
a^2+b^2=c^2
Slant height=c^2
Radius=b^2
So, a^2+6^2=16^2
       a^2+36=256
       a^2=220
       a=14.832397
Surface Area = πr(r+ square root of h^2+r^2)

Surface Area is approximately 414.69