Differential Equation - Integral Equation

Hello from Greece. Iam trying to solve and differential equation but cant write exactly the code. First of all below is the function

EM,ρL are contants and Vt1=k1*R so subtitute Vt1 in equation becomes:

The question is how to write the above differential to solve for k var.

[anestis]

the code is written for wolfram alpha

Answer 1

[Torsten]

syms r z k
eqn = int((20-1.19e8*r^2)/(1.19e8*r^2),r,30e-6,40e-6)==int(0.54*8*1e-3/4e3,z,0,k);
knum = solve(eqn,k)

knum = 

knum = double(knum)

knum = 1.2876e+03

[anestis]

Thanks a lot. i used integral in code instead int!!!