9. Solve the following system of three equations in three variables by determining the inverse of the matrix of coefficients and then using matrix multiplication. (10%)
\( x_1 + 2x_2 + 3x_3 = 1 \\ 2x_1 + 5x_2 + 3x_3 = 3 \\ x_1 + 8x_3 = 15 \)