I've been quite confused with these two questions, i'd be extremely grateful if anyone could answer and explain them to me!

F = GMm / r^2  for m :-

cross multiply:-

GMm = Fr^2

now divide both sides by GM:-

m =Fr^2 / GM

For  r :-

Fr^2 = GMm

r^2 = GMm / F

r = sqrt (GMm/F)  Answer

6.  Vt^2 = Vo^2 + 2ad

subtract 2ad from both sides:-

Vt^2 - 2ad = Vo^2

Vo  = sqrt (Vt^2 - 2ad)  answer

Yes you do, so £1973.09 - £1473.50 = £499.59 so tim overpaid £499.59 .hope this ; )