Off[General::"spell1"]
msteckou = 0.2;
ρ = 1000;
Vsteckou = (msteckou/ρ);
cp = 4186;
T1 = 4;
T2 = 45;
prikon = msteckou*cp*(T2 - T1);
cm = 0.01;
m = 50  cm;
d = 0.5  cm;
dosab = Solve[{a*20 + b == (1/70.) , 
              a*75 + b == (1/30.) }][[1]];
vx = (Vsteckou/(m*d));
gamma = a*T[x] + b /. dosab;
Ecko = (U/d);
U = 230;
rce = ρ*cp*vx*T'[x] == 
        gamma*(Ecko^2)
resT = DSolve[{rce, T[0] == 4}, T[x], 
          x][[1]]
teplota = T[x] /. resT

DSolve[{rce, T[0] == 4}, T[x], x]