My first thought was "tedious, yes, Calculating determinants always is." But, if you know what the "properties of determinants" are this is really very easy. You don't have to actually evaluate any 3 by 3 determinant!
We are given that \(\displaystyle \left|\begin{array}{ccc}5 & x & 7 \\ 10 & y & 3 \\ 20 & 17 & 7 \end{array}\right|= -385\). The first problem asks you to find the determinant \(\displaystyle \left|\begin{array}{ccc} 4 & 17 & 7 \\ 2 & y & 3 \\ 1 & x & 7\end{array}\right|\).
Do you see that the third column of this matrix is the same as the third column of the first? But since it is "symmetric" it is also the third column reversed? And that the second column is just the second column of the first reversed? And that the first column is one fifth the first column of the previous matrix, also reversed? So you can find the determinant of the second matrix without actually expanding it!
Of course, that will be an equation in x and y. And setting y= 12, you have an equation to solve for x.