For some reason I'm getting an error which says 'Set::write:'

I am trying to solve 3 simple differential equations but am new to Mathematica, could anyone let me know what i'm doing wrong?

Many Thanks,

All are constants apart from Z,Pb and R

Will

ClearAll["Global`*"];

Pl = rho*g*Z[t] + Patm

eqn1 = Pb[t] == rho*g*Z[t] + Patm + (2*gma)/R[t]

eqn2 = R[t] == (((1 + x)*(rho*g*Z[t] + Patm)*r0^3)/Pb[t])^(1/3)

eqn3 = Z'[t] = -(2/9)*(1/mu)*(R[t]^2)*(Pl - Pb[t]/(Rs*T))

eqn = {Pb[t] == rho*g*Z[t] + Patm + (2*gma)/R[t] &&

R[t] == (((1 + x)*(rho*g*Z[t] + Patm)*r0^3)/Pb[t])^(1/3) &&

Z'[t] = -(2/9)*(1/mu)*(R[t]^2)*(Pl - Pb[t]/(Rs*T)) ,

R[0] == 0.000025 && Z[0] == 1 }

(*eqns = {eqn1 && eqn2 && eqn3 , R[0] == 0.000025 && Z[0] == 1 }; *)

sol = NDSolve[eqn, {R[t], Pb[t], Z[t]}, t]