Start with the equation t = 1/2*sqrt(v). If t = 3, then we have 3 = sqrt(v)/2 so that (multiplying both sides of the equation by 2) we have 6 = sqrt(v). To get t without the square root around it, we need to remove the square root symbol by squaring both sides of the equation 6 = sqrt(v), getting 6*6 = 36 = sqrt(v)*sqrt(v) = v. So, the speed of the object that took 3 seconds to stop must have been 36 meters per second.