Hello, arturju!
Divideby0 has a good game plan,
. . and for this problem, it's not too difficult.
An isoceles triangle with a base of 24 and legs of 15
is inscribed in a circle. .Find the radius. Code:

B *(0,9)
**
*  *
*  *
*  o M
*  *
*  *
*  *
 A * * * * + * * * * C 
(12,0)  (12,0)
The midpoint of BC is: .M(6, 9/2)
The slope of BC is: .(0  9)/(12  0) .= .3/4
The perpendicular slope is: .m = 4/3
The equation of the perpendicular bisector is:
. . y  9/2 .= .(4/3)(x  6) . → . y .= .(4/3)x  7/2 .[1]
The perpendicular bisector of AC is the yaxis: .x = 0 .[2]
The intersection of [1] and [2] is: .P(0, 3½), the center of the circle.
The radius of the circle is: .PB .= .9  (3½) .= .12½