How do you find the derivative of
y=5sin(-x)
use the chain rule
y = f(u(x))
y' = df/du * du/dx
here, u = -x, so
dy/dx = 5cos(-x)(-1) = -5cos(-x)
which, since cos(-x)=cos(x), is -5cos(x)
Or, you can note that
sin(-x) = -sin(x), so what you have is
y = -5sin(x)
y' = -5cos(x)