The orbital period of an object is 2 × 107 s and its total radius is 4 × 1010 m.
the tangential speed of the satellite, written in standard notation, is
⇒ 12,560 m/s.

Tangential velocity (Vt) is the arc length (a) covered per unit of time (t):

Vt=a/t

Period (p) is the time needed to cover a complete circle, which length would be pi*2*radius (the same as the circle perimeter formula).

So you have to replace a for the perimeter and t for the period.

Vt=pi*2*r/p

Vt=pi*2*(4 × 10^10 m)/ (2 × 10^7 s)=12566m/s

(The number can vary a little with rounding – the difference is less than 1%).

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d.7.5cm is your answer