If ( a +3 b ) is perpendicular to (7 a -5 b ) and ( a -4 b ) is perpendicular to (7 a -2 b ), then the angle between a and b (in degrees) is \ \ \ \.

If $\displaystyle (\overrightarrow{\mathrm{a}}+3 \overrightarrow{\mathrm{~b}})$ is perpendicular to $\displaystyle (7 \overrightarrow{\mathrm{a}}-5 \overrightarrow{\mathrm{~b}})$ and $\displaystyle (\overrightarrow{\mathrm{a}}-4 \overrightarrow{\mathrm{~b}})$ is perpendicular to $\displaystyle (7 \overrightarrow{\mathrm{a}}-2 \overrightarrow{\mathrm{~b}})$, then the angle between $\displaystyle \overrightarrow{\mathrm{a}}$ and $\displaystyle \overrightarrow{\mathrm{b}}$ (in degrees) is \_\_\_\_.