Answer:
v = 5.5 m / s
Explanation:
For this exercise let's use Newton's second law, where the acceleration is centripetal
    F = ma
    a = v² / r
Before starting we reduce the mass to the SI system
    m = 250 g (1 kg / 1000g)
    m = 0.250 kg
Let's calculate the velocity for when the forces equal to T = 8.7N
    T = m v² / r
    v² = T r / m
    v = √ T r / m
    v = √ (8.7 0.87 /0.250)
    v = 5.5 m / s