The series $-4 + (-12) + (-20) + ... + (-740) + (-748)$ can be written using sigma notation (also called summation notation): $\large\displaystyle\sum\limits_{k=1}^{n}{{a_k}}$ **Write an expression for $a_k$ in terms of $k$.** $a_k=$ [[☃ expression 1]] **Find $n$.** $n=$ [[☃ numeric-input 1]]