By differentiating both sides of y = 4x² + 1 with respect to time t, we have
dy/dt = 8x dx/dt
If the x value changes at a rate of +2 units/second (that is, dx/dt = 2) at the moment x = 1, then the y value changes at a rate of
dy/dt = 8 • 1 • 2 units/s = 16 units/s
