When differentiating constant dy/dx is equal to what

A constant is a value that does not change. For example, if $y = c$, where $c$ is any real number (e.g., $y=5$, $y=-10$, $y=\pi$).

The derivative of a constant with respect to any variable is always zero because the rate of change of a value that does not change is zero.

Therefore, if $y$ is a constant, then $\frac{dy}{dx} = 0$.