RX is + XS is the hypotenuse of the right triangle RTS, then:
(RX + XS)^2 = (RT)^2 + (ST)^2
=> (4+9)^2 = (RT)^2 + (ST)^2
=> 13^2 = (RT)^2 + (ST)^2 .....equation (1)
Triangle RTX and XST are also right triangles.
RT is the hypotenuse of RTX and ST is the hypotenuse os SXT.
Then, (RT)^2 - (RX)2 = (TX)^2 and (ST)^2 - (SX)^2 = (TX)^2
=> (RT)^2 - (RX)^2 = (ST)^2 - (SX)^2
=> (RT)^2 - (ST)^2 = (RX)^2 -(SX)^2
=> (RT)^2 - (ST)^2 = 4^2 - 9^2 = 16 - 81 = - 65
=> (ST)^2 - (RT)^2 = 65 ..........equation (2)
Now use equations (1) and (2)
13^2 = (RT)^2 + (ST)^2
65 = (ST)^2 - (RT)^2
Add the two equations:
13^2 + 65 = 2(ST)^2
2(ST)^2 =178
(ST)^2 = 234/2 = 117
Now use (ST)^2 - (SX)^2 = (TX)^2
=> (TX)^2 = 117 - 81 = 36
=> (TX) = √36 = 6
Answer: 6
