Answer
a=b
a-c=b-c
c=d
a-c=b-d
Work Step by Step
a=b is given in the question.
Because a=b then if the same amount (c) is taken from both a and b, the two sides will still be equal. Therefore a-c=b-c.
c=d is given in the question.
If c=d then d can replace c in any equation, therefore b-c=b-d.
We already know that a-c=b-c, therefore a-c=b-d as well.
Therefore a-c=b-d is proved.
