Assuming a steady speed of 50 miles per hour,how long will it take to go 350 miles?

Divide.

I have to set up a proportion , and I don't know how.

How did they show you in school?

Set up two divisions with an equals sign in between.

Ergo:

3/4 = 6/8 is an example of a proportion.

When one of the four numbers in a proportion is unknown, cross products may be used to find the unknown number. This is called solving the proportion. Question marks or letters are frequently used in place of the unknown number.

x/4 = 6/8

Do the same with your problem.

By the way, just to add a little to what others said above, there are several keywords that you'll see in word problems throughout your school career that translate directly from english into "math". The word "per" is one of them. It means "divided by". For example, if your dog barks 42 times per day, that means (42 barks) \div (1 day). If you then wanted to know how many times your dog barks in 30 days, you'd recognize that the ratio of barks per day (i.e. # of barks divided by # of days) remains equal. Thus:

\frac{42 \; barks}{1 \; day}=\frac{x \; barks}{30 \; days}

Then you just have to solve for x.

The problem you have here is very similar, except instead of barks per day, you're dealing with miles per hour. Also, the unknown you're solving for will end up in the denominator in this case (it's hours your looking for, not miles). Otherwise, it's just like Wondergirl said.