8 6/18- (B+9/13) =4 2/13
8 6/18-(в+9/13)=4 2/13 -(в+9/13)=-8 6/18+4 2/13 в=8 6/18 - 4 2/13-9/13 В=8 6/18 -3 6/13 в=7 2/13