This is our initial concentration of our reactor. Now the slope represents negative k, this is our slope. So to find our slope, we simply find the final concentration, subtract that from initial concentration and divide it by the change in time, or M equals change in Y over change in X and we find our K value. And that's how we find our K.
Now, in other words, to summarize, if after graphing our concentration of reactants versus time, we get a negative slope like this, we get a graph like this. That means our A must be one. So now let's look at what the graph would look like if our A was actually one, if our order was first order or A equals one.