Your file says:

I need to find the X-axis coordinates where the trendline (red) meets the Z axis

That is quite different than the question you asked above. Your first question didn't indicate that you were showing a trendline, and you seemed to be looking for the y value at an arbitrary x value.

To answer the question you ask in your file, the trendline meets the Y axis (labelled Z on your chart) where x=0. That means you just need the constant value from the trendline equation. The quickest way to do this is look at the equation. Right-click on the trendline, select Format Trendline, then check the box for Display Equation on Chart. For Plot 1 that is:

y = -4207.1x^6 + 12509x^5 - 14934x^4 + 9110.8x^3 - 2979.8x^2 + 504.89^x - 32.291

So the trendline crosses the Y axis at -32.291.