Please reason step by step, and put your final answer within \boxed{}.
If $\overrightarrow {a}=(2,3)$, $\overrightarrow {b}=(-4,7)$, and $\overrightarrow {a}+ \overrightarrow {c}= \overrightarrow {0}$, find the projection of $\overrightarrow {c}$ onto the direction of $\overrightarrow {b}$.
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