We suspect you mean "geometric sequence". Multiply each term by r to get the next.
a1 = 3 . . . . . given
a2 = 3*2 = 6
a3 = 6*2 = 12
a4 = 12*2 = 24
a5 = 24*2 = 48
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A quadratic sequence is one in which second-differences are a constant. You have not provided enough information about the sequence so that more terms of a quadratic sequence could be found. While you may occasionally see problems asking you to write the formula for one of these, they are rarely studied in any detail in Algebra. At least three terms are needed before the next terms can be predicted.