Question 2893
1. Like regular numbers A+B =A implies B = 0 (zero matrix)

2. Not quuite clear tome. C=[10 15 -2 5] 
  let D=[2 -3 5 x]...
 Your CD may mean the dot product or CD^t(transpose) 
 = 10*2 + 15*(-3)+ (-2)* 5 + 5 x = 10
 = -35 + 5x =10, so x = 9.