Why does the ca shake when I do 70mph

There are a number of reasons a car might shake while at highway speeds. Improper tire balance is usually the most common cause. However, you could also have some poor steering and suspension components that are causing the problem. Lastly, you could have improper tire wear. Plus, the term "shake" can mean a lot of different things in the auto world. Best thing to do is get it to a reputable shop and let them diagnose it for you and then you can know exactly what the problem is.