20sqrt (2) = sqrt (800)
Using the distance formula we have that
( x - 7)^2 + (21 - 4)^2 = (sqrt (800) )^2
( x - 7)^2 + 289 =800
(x -7)^2 = 511 take both roots
x - 7 = sqrt (511) x - 7 = - sqrt (511)
x = 7 + sqrt (511) x = 7 - sqrt (511)
Sum of all possible values of x = 14