I x + 3 I = +ve value but notice there is a negative on the left of it - I x + 3 I so it will be negative.
So we could write it by isolating the second absolute value
IxI + 3 -x-3=6
Now add 3 to both sides
(1) solve it, -2x=-6
x=-3 (Check and it works!)
(2) solve it, 2x=6
x=3 (Check and it doesn't work I.e extranous solution)