How do u know if a problem is arithmetic or geometric?

The defining characteristic of an arithmetic sequence is called the common difference.  This just means that the difference between each term and the previous term is a constant.  It has the form:

a(n)=a+d(n-1), a=initial term, d=common difference, n=term number

The defining characteristic of a geometric sequence is called the common ratio.  This means that the quotient between each term and the previous term is a constant.  It has the form:

a(n)=a*r^(n-1), a=initial term, r=common ratio, n=term number.

So in even simpler terms, an arithmetic sequence is a linear sequence and a geometric sequence is an exponential sequence.