This is the basic and conceptual question from Chapter name- circles
Topic – Angle properties of circles
Chapter number- 15
We have been given the length of the direct common tangent to two circles of radii 12 cm and 4 cm is 15 cm.
And we have to calculate the distance between their centres
ICSE Avichal publication
Understanding ICSE Mathematics
Question 14
Solution:

Let R and r be the radii of the circles
with centre A and B respectively
Let TT’ be their common tangent.
Hence distance between their centres = 17 cm Ans.