13,328.73
To find this, first you need to find the radius which can be found through the pythagorean theorem set up as so:
24^2 + b^2 = .51^2
This gets you 44.45cm.
You then plug this in to the surface area equation of a cone which is:
SA = pi x radius (radius + square root of h^2 + r^2 unsquare root)
SA = pi x 44.45cm ( 44.45cm + square root of 25cm^2 + 44.45cm^2 unsquare root)
SA = 13,328.73cm^2
Hope this helps!