To find the gradient of a line you use this equation: Rise / Run
I am assuming this is a graph where both the x and y-axis increase in value by one.
So first of all, you should draw out this graph.
Second, draw a point at each of the given coordinates.
Now, join these points by drawing a right angle triangle. Put simply, draw a line from the point (4, -7) down until it is on the same level as the point (2, -3), then draw a line across.
Finally, measure the length of both these sides and use them in the equation above.
Let's assume the rise (vertical line) and the run (horizontal line) are 5 and 8 respectively. We can do 5/8 to get a gradient which is 0.625.