When reflecting across the y-axis, you will want to switch the sign of the x-coordinate. If the x-coordinate is positive, you will want to switch it to negative and vice versa.
Examples:
(3,5) --> (-3,5)
(-6,-2) --> (6,-2)
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If you are going to reflect across the x-axis, you will change the y-coordinate, so it is the opposite of reflecting across the y-axis.