To answer this question we have to draw a graph:
the initial point is (-1;3), the reflection is on x =4.
Since it is reflected across an x line, the y component stays the same.
The distance from the line of reflection (x=4) to the image it's equal to the distance from the line of reflection to the initial point.
In this case, that distance is 5 units on the x-axis.
So, the new coordinates are:
(-1+5+5,3)= (9,3)
