Since your solving for d, its actually quite simple to figure out.
First:
9.7 = 1.2 + d + 0.4
Add 1.2 and 0.4 together to get 1.6
1.2 + 0.4 = 1.6
Then, subtract 1.6 from 9.7
9.7 - 1.6 = 8.1
Finally, fill in the blank for d
1.2 + 8.1 + 0.4 = 9.7
so, d = 8.1
Hope that helps you!