I am going to attempt to draw this...please bear with me :P

When two chords intersect in a circle, the products of their segments are equal.

Thus we have   ....  20 * 15 = 26 * x      ....    so we have

300 = 26 * x         divide both sides by 26

300/26 = x  = 150/13 =  about 11.538

Thank so much! That helps me alot.

:)