Given
y = 5x² - 1
differentiating both sides with respect to time t yields
dy/dt = 10x dx/dt
We're told that y decreases at a rate of 2 units/s, which means dy/dt = -2. At the moment x = 1, we have
-2 = 10•1 dx/dt ⇒ dx/dt = -2/10 = -1/5
which is to say, the x value decreases at a rate of 1/5 units/s.
