Answer
See below
Work Step by Step
Matrix $C$ is obtained by multiplying the second row of matrix $A$ with 3 and adding the first one to the last one. Then we need to find the determinant of matrix $C$.
Let's call the matrix that results from multiplying the second row of matrix $A$ with 3 $A_1$. Then we have $det(A_1)=3\det A=3(3)=9$
Let's add the first row to the last row. Matrix $C$ is the resultant matrix. The property of the determinants dictates that
$\det(C)=\det(A_1)=9$.
As a result, the determinant of the matrix $C$ is $\det (C)=9$