14d + 21 and
7(2d + 3)

D can be any value

Explanation:
First, we apply the distributive property to 7(2d+3) by multiplying 7 by both values (this is to open up the parentheses)

7(2d+3)
=14d+21

Now, we learn that 7(2d+3)=14d+21, which is the same thing as 14d+21=14d+21.

14d+21=14d+21
14d-14d=21-21
0=0

Because we’ve reached 0=0, which is a true statement, and not a value for d, d can be any value and the statements will be true.

I hope this helps!