How to find thedirectional derivativeof http://hosted2.webwork.rochester.edu...e1323b7171.png at the point (-1, -4) in the direction http://hosted2.webwork.rochester.edu...2e9f221871.png?

i already find the gradient of f at (-1,-4) = 160i+50j , but how can i find the directional derivative using http://hosted2.webwork.rochester.edu...2e9f221871.png?