Answer:
7.00cm (3sf)
Explanation:
The total surface area of a cone is given by πr² + πrl, where r is the radius and l is the slant height. We can use the info in the question to form an equation as follows:

We can see that this is a quadratic equation in terms of r, so to solve it we bring all terms to one side and use the quadratic formula

Plugging this into a calculator we get 2 solutions: r = -17.00 or r = 7.00
However as this is a length we only want positive solutions so we discard the negative root.