\(\begin{vmatrix}a&h&g\\h&b&f\\g&f&c\end{vmatrix}=a\begin{vmatrix}b&f\\f&c\end{vmatrix}-h\begin{vmatrix}h&f\\g&c\end{vmatrix}+g\begin{vmatrix}h&b\\g&f\end{vmatrix}\)
= a(bc – f2 ) – h(hc — gf) + g(hf- gb)
= abc – af2 – h2 c + fgh + fgh – g2 b
= abc + 2fgh – af2 – bg2 – ch2
= (-15) – (-4)(7)
= -30 + 28
= -2