Answer:
31
Step-by-step explanation:
In circle S: XB and XD are tangents at B and D
then SB perpendicular to AX and SD perpendicular to XC
In triangle SDX: SD = 12 and SX =20 by using Pythagoras Theorem
XD = XB tangents to circle S from point X
XA = XC tangents to circle R from point X
then BA = DC = 15
then XC = XD + DC = 16 + 15 = 31
