Let's solve this problem graphically. Here we have the following equation:
[tex]sin^{-1}(4x) + sin^{-1}(3x) = -\frac{\pi}{2}[/tex]
So we can rewrite this as:
[tex]f(x)=sin^{-1}(4x) + sin^{-1}(3x) \\ \\ g(x)= -\frac{\pi}{2}[/tex]
So the solution to the equation is the x-value at which the functions f and g intersect. In other words:
[tex]f(x)=g(x) \\ \\ sin^{-1}(4x) + sin^{-1}(3x) = -\frac{\pi}{2}[/tex]
Using graphing calculator, we get that this value occurs at:
[tex]\boxed{x=-0.2}[/tex]