A deuteron (Z = 1, A = 2) and a triton (Z = 1, A = 3) must touch before the nuclear force can act…

Step 1. The distance. $R_D = 1.2\ \text{fm} \times 2^{1/3} = 1.512$ fm and $R_T = 1.2\ \text{fm} \times 3^{1/3} = 1.731$ fm. So $d = 1.512 + 1.731 = 3.243$ fm.

Step 2. The barrier. $U = \dfrac{1.44\ \text{MeV fm} \times 1 \times 1}{3.243\ \text{fm}} = 0.444$ MeV.

Step 3. The temperature. We want $T$, so $T = \dfrac{2U}{3k} = \dfrac{2 \times 0.444 \times 10^{6}\ \text{eV}}{3 \times 8.617 \times 10^{-5}\ \text{eV/K}} = 3.44 \times 10^{9}$ K.