req. help with solving eqn, 2.0 Prime

Hello,

I'm brand new to MathCad, and seem to be stuck where the calc will not converge, so I'm not getting an answer... the eqn comes from my course notes.

I'm trying to solve to give the volume of 3 seperate sections of an object, starting with the first eqn, yfl(x).  This is supposed to be the "radius" when I sub in into Eqn 3 below to calculate the volume integral.

Where Lf= 86.4, nf = 2.25 and D=36, as seen in the MathCad worksheet.

Thank you