Determine impact of historical data on current data

Hello, everyone,

I am fairly new to this forum and also new to statistics in general. I learned a little about probabilities, statistics, anova, regression but i don't really understand how to apply them to determine the correlation between the last year advertising effects and current year sales performance. Let me give you more details on what i want to study:

So, i have an ecommerce website that is 7 years old and we used google/cpc as advertising method since begin.
I want to demonstrate that the Google/cpc positively influenced another sources of traffic based on following metrics: users / transactions (or an qualitative metric like transaction rate or transaction per user).

My assumption is that i should run an correlation test on the y-1 year performance in Google/CPC and y year performance in general statistics of all other sources except google/cpc. My purpose is to determine an equation like generic = x*google-cpc + y*organic-traffic etc.

What do you think? Is my assumption good? will the result be relevant? How i apply this?

Sorry for my bad english and thank-you in advance!