Let me show u one in detail.
First we have to find the common ratio. We do this by dividing the second term by the first term.
ur first problem...
12/3 = 4...so the common ratio is 4
now we use the formula
an = a1 * r^(n-1)
n = term u want to find = 15
a1 = first term = 3
r = common ratio = 4
now we sub
a15 = 3 * 4^(15 - 1)
a15 = 3 * 4^14
a15 = 3 * 268435456
a15 = 805,306,368 <== ur 15th term
thats all there is to it...and, of course, u can always look on the internet for a geometric sequence calculator...to check urself...lol