Vector Algebra

In a triangle ABC, if \(\left| {\overrightarrow {BC} } \right| = 3\), \(\left| {\overrightarrow {CA} } \right| = 5\) and \(\left| {\overrightarrow {BA} } \right| = 7\), then the projection of the vector \(\overrightarrow {BA}\) on \(\overrightarrow {BC}\) is equal to :