Let the price be p and the quantity q.
Assume a linear relationship between q and p: q=mp+a where m and a are constants.
So 250=2m+a when the price was $2 per widget and 175=3m+a when the price was $3.
Now, subtract one equation from the other:
75=-m, so m=-75. Therefore, 250=-150+a and a=400.
So the relationship is q=400-75p.