That's not a special angle, so you'll have to use a calculator. If you have a TI graphing calculator, Make sure it's in RADIAN mode: Press MODE Scroll down to the third line RADIAN DEGREE. Make sure RADIAN is highlighted. Press 2ND then MODE to get back to the main screen. Press CLEAR Press 2ND then TAN, you'll see tan-1( Type 3/7) after that so you see tan-1(3/7) Press ENTER Read .4048917863 Edwin