When the value of x increases from 0 to 2, the value of the function decreases by 4, then the analytic expression of the parabola y = ax & # 178; - 1 is?

When the value of x increases from 0 to 2, the value of the function decreases by 4, then the analytic expression of the parabola y = ax & # 178; - 1 is?

When x = 0, the function value is Y1 = - 1
When x = 2, the value of function is y2 = 4a-1
Then:
y2-y1=-4
(4a-1)-(-1)=-4
a=-1
We get: y = - X & # 178; - 1