What is the slope of the line through the points (–3, 5) and (4, 5)

Slope is referred to as m in y=my+b form and m=y2-y1/x2-x1
In this particular problem with the given coordinate points of (-3,5),(4,5) your numerator(or y2-y1 value) will be 0 and 0 divided by any real number is 0, so the the slope is zero.

You can visualize this as a straight line going through the y intercept at y=5

Hope this helps understand!