Answer: Hello!
our point is P(0,0) at the beginning, and then we do a reflection across x= -3.
this reflection only affect the x, the initial distance between x and -3 is of 3 units, then when we do the reflection we took this distance and translate it to the other side of the axis -3.
then we go from x = 0, to x= -6, where the distance between - 6 and -3 still is for 3 units, but now for the other side.
When we do the reflection over the axis at y = 4, we think in the same way, the x part remains unchanged, it only affects the y part.
then the distance between y and 4 is for 4 units, then y translates by 4 units to the "other side" of 4, this is 4 + 4 = 8, after the reflection y is equal to 8.
so the point (0,0) is now (-6,8)
is kinda hard to explain this without a drawing, if you have any problem understanding it, please write to me.