when you reflect something over the y axis you are just changing the x value from + to negative . For example +2 becomes -2 so in your equation change x to -x and you get
y= 2(-x) -4 or y= -2x-4
I will let you try th second one
When you reflect over the x axis you change the y value from + to negative so +3 becomes -3
Now in the given equation change y to -y and you get
-y= 3x-5 multiply both sides by (-1) and you get y =-3x+5
Now you try the second one