Assume that X should be subtracted from (-11/15) to get (-3/15).
So ,
( -11/15) - x = -(3/15).
take x to right side and (-3/15) to left side.then their sign will change also.
(-11/15)+(3/15) = x
(-11+3)/15 = x
X = -8/15.
So -8/15 should be subtracted.