msteckou = 0.2;
ρ = 1000;
cp = 4186;
Vsteckou = (msteckou/ρ);
T1 = 4;
T2 = 45;
prikon = msteckou*cp*(T2 - T1);
vx = (Vsteckou/(m*d));
resab = Solve[{20*a + b == (1/70.) , 
              75*a + b == (1/30.) }][[1]];
U = 230;
m = 0.5;
cm = 0.01;
d = 0.5  cm;
Ecko = (U/d);
rce = ρ*cp*vx*T'[x] == (a*T[x] + b)*(Ecko^2) /. 
          resab;
poc = T[0] == T1;
res = DSolve[{rce, poc}, T[x], 
            x][[1]];
teplota = T[x] /. res;
Plot[{teplota, T2}, {x, 0, 0.5}, 
      AxesLabel -> {"x[m]", "T[°C]"}, 
      PlotStyle -> {Thickness[0.008], Thickness[0.008]}]