find the convariance from stock a and b

stock a$=(4\mathrm{.}9,6\mathrm{.}2,-1\mathrm{.}5,2\mathrm{.}1,-5\mathrm{.}4,6\mathrm{.}8)$

stock b$=(1\mathrm{.}8,9\mathrm{.}6,-3\mathrm{.}8,-5\mathrm{.}9,6\mathrm{.}7,-7\mathrm{.}4)$

period$=(1,2,3,4,5,6)$