There seems to be a missing round bracket. If it was supposed to be 104=1/2[(360-x)-x], then open up the round brackets: 104=1/2[360-x-x], 104=1/2[360-2x], double both sides: 208=360-2x; add 2x to each side: 208+2x=360 and subtract 208 from each side: 2x=152, so, halving both sides gives x=76.