Answer
13 m/s.
Work Step by Step
The net horizontal force, the tension, is the centripetal force that causes the circular motion.
$$F_{net}=75 N = ma_R=\frac{mv^2}{r}=\frac{(0.55 kg)(v)^2}{1.3 m} $$
$$v=\sqrt{\frac{(75 N)(1.3 m)}{0.55 kg}} \approx 13 m/s$$
The answer is displayed to 2 significant figures.