Let \( a_{1}, a_{2}, a_{3}, \ldots \) be in an arithmetic progression of positive terms.
Let \( A_{k} = a_{1}^{2} - a_{2}^{2} + a_{3}^{2} - a_{4}^{2} + \cdots + a_{2k-1}^{2} - a_{2k}^{2} \) If \( A_{3} = -153 \), \( A_{5} = -435 \) and \( a_{1}^{2} + a_{2}^{2} + a_{3}^{2} = 66 \), then \( a_{17} - A_{7} \) is equal to ____________.
