Clear["Global`*"]
Ac = 1.98 e - 6;
k = 6.7;
P = 0.005;
U = 4;
Cp = 0.5263;
q = 300;
[Sigma] = 5.6703*10^-8;
[Epsilon] = 0.5;
To = 295;
Ti = 295;
Tl = 2273;
h = 0.0025;
l = 0.005;
Ta = 500;

eqns = {(Ac*k)*x''[t] - (P*Ac*U*Cp)*x'[t] + P*q ==
0, (Ac*k)*y''[t] - (P*Ac*U*Cp)*y'[t] - [Sigma]*[Epsilon]*
P*(Ta - Ti)*(Ta^2 - Ti^2) y[t] == 0, x[0] == To, y[l] == Tl,
x[h] == y[h], x'[h] == y'[h]};

s = DSolve[eqns, {x[t], y[t]}, t];

y[t] /. s;
N[y[t] /. s /. t -> 30]