Answer
plane in still air: 452.6 mph
wind speed: 52.6 mph
Work Step by Step
with wind: $(x+y)*4.75=2400$
into wind: $(x-y)*6=2400$
$(x+y)*4.75=2400$
$4.75x+4.75y=2400$
$(x+y)*4.75=2400$
$(x+y)*4.75*4=2400*4$
$(x+y)*19=9600$
$(x-y)*6=2400$
$(x-y)*6/6=2400/6$
$x-y=400$
$x-y+y=400+y$
$x=400+y$
$(x+y)*19=9600$
$(400+y+y)*19=9600$
$(400+2y)*19=9600$
$7600+38y=9600$
$38y=2000$
$38y/38=2000/38$
$y=52.632$
$x=400+y$
$x=400+52.632$
$x=452.632$