Let us label two circles as 'a' and 'b

As TQ and TP are tangents to circle a,And TP and TR are tangents to circle b.
By theorem which states that the lengths of the two tangents drawn from external point to a circle are equal.
TQ=TP ...(1)
TP=TR ...(2)
From 1 and 2,
TQ=TR
Hence proved