Simplify the given mathematical expression: $(x+3)+\left(9 x^{2}+7 x\right)$

Step 1 ：Given the mathematical expression: \( (x+3)+\left(9 x^{2}+7 x\right) \)

Step 2 ：We can simplify this expression by combining like terms. The like terms in the expression are the terms that have the same variable and exponent.

Step 3 ：In this case, the like terms are \(x\) from \(x+3\) and \(7x\) from \(9 x^{2}+7 x\). We can add these together.

Step 4 ：The \(9x^2\) term does not have a like term, so it will remain as it is.

Step 5 ：By combining the like terms, the expression simplifies to \(9x^2 + 8x + 3\).

Step 6 ：Final Answer: The simplified expression is \(\boxed{9x^2 + 8x + 3}\)