Sequences And Series

Let \(\{ {a_k}\}\) and \(\{ {b_k}\} ,k \in N\), be two G.P.s with common ratios \({r_1}\) and \({r_2}\) respectively such that \({a_1} = {b_1} = 4\) and \({r_1} < {r_2}\). Let \({c_k} = {a_k} + {b_k},k \in N\). If \({c_2} = 5\) and \({c_3} = {{13} \over 4}\) then \(\sum\limits_{k = 1}^\infty {{c_k} - (12{a_6} + 8{b_4})}\) is equal to __________.