Your questions seem to be truncating, but I can answer this one.....
Using the distance formula....
sqrt [ (2a - 4)^2 + (a - 4 - (-1) )^2 ] = 2sqrt (10) square both sides
(2a - 4)^2 + ( a - 3)^2 = 40 simplify
4a^2 - 16a + 16 + a^2 - 6a + 9 = 40
5a^2 -22a -15 = 0 factor
(5a - 3) ( a + 5) = 0
So
5a - 3 = 0 a + 5 =0
5a = 3 a = -5
a = 3/5
Product of all possible values of a = (3/5) (-5) = -3