A sum of an geometric sequence is a(1-r^n)/(1-r) where n=15 in this problem and r=4 since each term is being multiplied by 4 to get the next term. a also equals 4 since that is the first number. So now you have 4(1-4^15)/(1-4) which you can type into your calculator. Be sure to use the parenthesis so the calculator will know what is in the numerator and what is in the denominator,