How to solve this equation? Thanks

Which equation and solution can be used to solve this problem? thirteen less than a number is sixteen. 13-n=16: add 13 to both sides. the answer is 29. n+13=16: subtract 13 from both sides. the answer is 3. n+16=13 subtract 13 from both sides. the answer is 3. n-13=16: add 13 to both sides. the answer is 29.

If given an equation of a line such as -1/2x+6 how would you create an equation of a line parallel and perpendicula to this line that goes through another point such as (4,10)

Apinhole camera is made out of a light-tight box with a piece of film attached to one side and pinhole on the opposite side. the optimum diameter d(in millimeters) of the pinhole can be modeled by d=1.9[ (5.5 x 10^-4) l) ] ^1/2, where l is the length of the camera box (in millimeters). what is the optimum diameter for a pinhole camera if the camera box has a length of 10 centimeters