Suppose $\overrightarrow {a}= \lambda \overrightarrow {i}- 7 \overrightarrow {j}+3 \overrightarrow {k}, \overrightarrow {b}= \lambda \overrightarrow {i}+\overrightarrow {j}+2 \lambda \overrightarrow {k}$. If the angle between $\overrightarrow {a}$ and $\overrightarrow {b}$ is greater than $90^{\circ},$ then $\lambda$ satisfies the inequality: