To find the 1st difference given the second term and third term of a sequence, we can use the formula for the 1st difference, which is calculated as:
\[ \text{1st difference} = \text{third term} - \text{second term} \]
Given the second term as \(3a + 5\) and the third term as \(8a - 3\), we can calculate as follows:
\[ \text{1st difference} = (8a - 3) - (3a + 5) \]
Now, simplify the expression:
\[ \text{1st difference} = 8a - 3 - 3a - 5 \]
Combining like terms:
\[ \text{1st difference} = (8a - 3a) + (-3 - 5) = 5a - 8 \]
So, the 1st difference is:
\[ \text{1st difference} = 5a - 8 \]
The blank spot can be filled with \(5a - 8\).