Let $f(x) =x^3-7x-2$ and $g(x)=-0.5\cdot 3^x+3$. The graphs of $y=f(x)$ and $y=g(x)$ are shown below. The point $(a,b)$ is on the graph of function $f$.  **The value $x=a$ is a solution to which of the following equations?** [[☃ radio 1]]