7x - 3 - n = 3x + 4
move the expression to the right:
-n = 3x + 4 - 7x + 3
collect like terms:
-n = -4x + 4 + 3
add the numbers:
-n = -4x + 7
change the signs:
n = 4x - 7
so, 4x - 7 must be subtracted to 7x - 3 to obtain 3x + 4.
(7x - 3) - (4x - 7) = 3x + 4
