to be arithmetic then you have to 'add' the same number each time to get the next value, call this number x
then (y+4)+x=6 and 6+x=y
solve the 2nd equation for x and x=y-6
substitute this into the first equation for x and (y+4) +(y-6)=6
combine like terms and 2y-2=6
add 2 to both sides and 2y=8
divide both sides by 2 and y=4