What is 12/15 = ab/20
(geometry converse of pythagorean theorem)

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

this may not be at all, but i use the same website you're using for school, and if i'm stuck doing one of these i try to look for it on the section.