There are two ships , let them be ship A and ship B.
Let X be the distance of ship A from the light house .
tan 4 =350 / X
Let Y be the distance of ship B from the light house
tan 83.5 = y/350
I got 83.5 from 90-6.5
x-y = answer
I assume 'directly offshore' as both ships are offshore in the same path or if i intepreted the question wrongly , the above working would not be valid . .