when i know the y value of the vertex, how do i find the x value?

If you know the y-coordinate of the vertex of a quadratic function, you can use the vertex form of the function to find the x-coordinate of the vertex. 
 The vertex form of a quadratic function is: 
 y = a(x - h)^2 + k 
 where (h, k) is the vertex of the parabola. 
 If you know the y-coordinate of the vertex, which is k in the vertex form equation, you can substitute it into the equation to get: 
 y = a(x - h)^2 + k 
 Simplifying this equation, we get: 
 y - k = a(x - h)^2 
 Dividing both sides by "a", we get: 
 (y - k) / a = (x - h)^2 
 Taking the square root of both sides, we get: 
 √((y - k) / a) = x - h 
 Adding "h" to both sides, we get: 
 x = h ± √((y - k) / a) 
 So, to find the x-coordinate of the vertex, you can use the formula: 
 x = h ± √((y - k) / a) 
 where "h" is the x-coordinate of the vertex, "k" is the y-coordinate of the vertex, and "a" is the coefficient of the x^2 term in the quadratic function.