Find a quadratic function of the form $y=a x^{2}$ that passes through the point $(3,-18)$. \[ y= \] (Use integers or fractiofis for any numbers in the expression.)

Step 1 ：Substitute the x and y values of the point into the equation \(y = ax^2\) to get \(-18 = a(3)^2\).

Step 2 ：Solve the equation to find the value of a, which is -2.

Step 3 ：\(\boxed{y = -2x^2}\) is the quadratic function that passes through the point (3,-18).