Answer
off the walkway: 4 ft/s
speed of walkway: 2 ft/s
Work Step by Step
groundspeed is 2 f/s against walkway, 6 f/s with walkway
$x-y = 2$
$x+y = 6$
$(x-y)+(x+y) = (2+6)$
$2x = 8$
$2x/2 = 8/2$
$x= 4$
$x-y=2$
$4-y=2$
$4-y-2+y = 2-2+y$
$2 = y$