You can get the of if you apply Newton's Second Law before the biker brakes. What are the forces on the biker before he brakes, and what is the acceleration?
Incidentally, I would think that a gradient of would imply that the angle of inclination of the hill is given by not
I don't think there's a way to get an exact numerical answer without finding , but as I've said, I think there's a way to find .
Can you see your way forward now?