#### Answer

Fill the blanks with
... 7 ... 7 ... 10
... 5 ... 10 ... 5
... 3 ... 1 ... 15

#### Work Step by Step

Rightmost column: 3 is less than 5 in any base,
borrow one group of 7, giving (7+3$)=10_{ten}$.
Then subtract 5 from 10, obtaining a difference of 5
Record the 5 (we have borrowed one 7)
$\begin{array}{lll}
& & \\
& 4 & 3_{seven}\\
- & 2 & 5_{seven}\\
-- & -- & --\\
& & 5_{seven}
\end{array}$
Move onto the next column.
We subtract 2 from 3 (not 4, one was borrowed)
and obtain 1.
Record the one, the answer is $15_{seven}$
$\begin{array}{lll}
& & \\
& 4 & 3_{seven}\\
- & 2 & 5_{seven}\\
-- & -- & --\\
& 1 & 5_{seven}
\end{array}$
Fill the blanks with
... 7 ... 7 ... 10
... 5 ... 10 ... 5
... 3 ... 1 ... 15