The fastest speed before the string breaks is 9.5 m/s
Explanation:
The motion of the block is a uniform circular motion, which is a circular motion with constant speed. The force that keeps the block in circular motion is called centripetal force; its direction is towards the centre of the circle and its magnitude is given by:

where
m is the mass of the block
v is its speed
r is the radius of the circle
In this problem, the centripetal force is provided by the tension in the string, T, so we can write:

The string breaks when the centripetal force becomes larger than the maximum tension in the string:

Re-arranging the equation for v,

and here we have:
T = 450 N
m = 10 kg
r = 2 m
Substituting,

So, the fastest speed before the string breaks is 9.5 m/s.
Learn more about circular motion:
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