What are the coordinates of y(-3.5,5) across the x-axis?

I'm assuming you are asking what the coordinates would be if they were reflected across the x-axis. 

The answer to that is (-3.5, -5). When you reflect across the x-axis, the y-coordinate will change signs. And when you reflect across the y-axis, the x-coordinate will change signs.

All you have to do is multiply