Let a_1, a_2, a_3, … be in an arithmetic progression of positive terms. Let A_k = a_1^2- a_2^2+ a_3^2- a_4^2+…+ a_2 k -1^2- a_2 k^2. If A_3=-153,…

Let $\displaystyle \mathrm{a}_1, \mathrm{a}_2, \mathrm{a}_3, \ldots$ be in an arithmetic progression of positive terms. Let $\displaystyle \mathrm{A}_{\mathrm{k}}=\mathrm{a}_1^2-\mathrm{a}_2^2+\mathrm{a}_3^2-\mathrm{a}_4^2+\ldots+\mathrm{a}_{2 \mathrm{k}-1}^2-\mathrm{a}_{2 \mathrm{k}}^2$. If $\displaystyle \mathrm{A}_3=-153, \mathrm{~A}_5=-435$ and $\displaystyle \mathrm{a}_1^2+\mathrm{a}_2^2+\mathrm{a}_3^2=66$, then $\displaystyle \mathrm{a}_{17}-\mathrm{A}_7$ is equal to $\displaystyle \_\_\_\_$.