Question 43162
Solving by backtracking is just doing the inverse operations at the inverse order. If we normally divide and add, to backtrack we will subtract and multiply. Algebraically meaning:
{{{end = start/2 + 2}}}
{{{-start/2 = 2 - end}}}
{{{-start = 4 - 2*end}}}
{{{start = 2(end-2)}}}

For end = 8, subtract 2 and multiply by 2:
{{{s = 12}}}

For end = 12
{{{s = 20}}}

For end = 20
{{{s = 36}}}