The equation of a hyperbola in standard form is a2x2−b2y2=1 and its foci are (±ae,0) and directrices are x=±ea.
The given equation of hyperbola is 16x2−9y2=144,⇒9x2−16y2=1
⇒a2=9,b2=16
Given, directrix of the hyperbola is 5x+9=0,⇒x=−59.
Hence, on comparing the directrix with the standard equation, we get ea=59
⇒e3=59
⇒e=35
And, hence the required focus is (−ae,0)=(−5,0).