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1) common difference is 6 - try $\displaystyle a_n = 6n+$k and find k that works.
2) Dividing shows the same number - 5, try $\displaystyle a_n = c_1 5^n + c_2$
3) the difference is 8, 12, 16, 20 and then again is 4 which suggests a formula of the form $\displaystyle a_n = pn^2+qn+r$ - find p,q and r.