Mathematicians in Space | Black Hole Brainteasers: Problem 1

Mathematicians in Space is an elite group of problem solvers who have mastered the art of brainteasers. They're not just solving for x, they're solving for the very fabric of space and time.

Here's your first brainteaser, straight from the depths of a black hole:

Problem 1: The Event Horizon Conundrum

A black hole with a mass of 10^6 M⊂ is rotating at 0.99c. What's the radius of the event horizon?