Question: how can I solve an equation?

Hi all
how can I solve an equation? Pleas, help!

I have a differential equation with one unknown.
I first solve the differential equation, and then use the boundary condition to find his unknown, however, get the error:
Error, (in sol_2) parameter 'X' must be assigned a numeric value before obtaining a solution.

How can I find X?

 F:=1:
 l:=1:
 a:=0.1:
 n:=4.5:

 B:=1.47*10^(-11):
 J_nc:=evalf(2*a*int(y^(1/n+1),y=0..a/2)):
 M_f:=z->piecewise(`and`(z>=0,z<=l),F*l,`and`(z>l,z<=2*l),F*l-F*(z-l),`and`(z>2*l,z<=3*l),F*l-F*(z-l)+F*(z-2*l));
 M_ed:=z->z-3*l:
 M_x:=z->M_f(z)+X*M_ed(z);
 eq := diff(V(z), `$`(z, 2)) = B*(abs(M_x(z))/J_nc)^n*signum:cond := V(0) = 0, (D(V))(0) = 0;
 sol_2 := dsolve({cond, eq}, numeric);
 sol_3:=solve({sol_2(3*l)=0},{X});