To simplify the given expression, we need to distribute the negative sign through the second set of parentheses and then combine like terms.
Step 1: Distribute the Negative Sign
We start with the expression:
\[
(7u^{2} + 3) - (2u^{2} - 6u + 4)
\]
Distributing the negative sign through the second set of parentheses gives us:
\[
7u^{2} + 3 - 2u^{2} + 6u - 4
\]
Step 2: Combine Like Terms
Next, we combine the like terms:
For \(u^{2}\) terms: \(7u^{2} - 2u^{2} = 5u^{2}\)
For \(u\) terms: \(6u\) (there are no other \(u\) terms to combine with)
For constant terms: \(3 - 4 = -1\)
Thus, the expression simplifies to:
\[
5u^{2} + 6u - 1
\]
Final Answer
The simplified expression is
\[
\boxed{5u^{2} + 6u - 1}
\]