A regular tetrahedron is a triangularbase edges of 7 cm?

pyramid. What is the total surface

area of a regular tetrahedron with

I started by using

1/2 × Perimeter × [Side Length] + [Base Area]

(1/2) (21) (?) + (sqrt(36.75))

So im not sure if this is the correct forumula or if the numbers i used to plug in are correct.