TeX source:

\begin{align*} d(2,4) &= 1 + d(2, 4.from) = 1 + d(2, 6)\\ &= 2 + d(2, 6.from) = 2 + d(2, 7)\\ &= 3 + d(2, 7.from) = 3 + d(2, 2)\\ &= 3 \end{align*}