A = 2/7-3 = 2/7 - 3/1 = 2/7 - (3*7)/(1*7) = 2/7 - 21/7 = (2-21)/7 = -19/7
B = -11/4+7/16 = (-11*4)/4*4 + 7/16 = -44/16 + 7/16 = -37/16
C = -5/14+-10/21 = -5*3/14*3 + (-10*2)/(21*2) = -15/42 + -20/42 = -35/42 = -5/6
D = 15/42:25/48 = 15/42 * 48/25 = 720/1050 = 72/105 = 24/35