no idea how to start, please help!

Recall that a DNA sequence is a string over the alphabet {A,C,G,T}. The "GC-Content" of a DNA sequence is the proportion of characters of the sequence that are either G or C. For example, the GC-Content of the sequence ACGTA is 2/5 = 0.4. TASK: Write a static method called gcContent with one String parameter sequence. It should return the GC-Content of sequence as a double. Sample Input: ACGCGCGCCT Sample Output: 0.8