|your trendline will give you a "line of best fit". Make sure you're using a trendline and NOT just the scatter graph with smoothed lines joining point to point.|
So you'll just have a series of unconnected points. Right click on one of them and "Add trendline". It will default to linear, then find the option to display equation on line. Should be sorted.
If this doesn't work, use maths instead.
Gradient = change in y divided by change in x. So if your values are
Your gradient is 3 divided by 4, or 0.75