The key points to note in this question are 'Rhumb Line Track' and 'approximate'. You are given no track direction information, so you cannot apply Conversion Angle to convert between Great Circle and Rhumb Line Track; you are not expected to apply 3-dimensional spherical trigonometry; so, there MUST be a way to approximate.
First, draw the problem out as if it were on a Direct Mercator (straight and parallel Meridians), because then a straight line represents a Rhumb Line. If you work out the N/S distance, it comes to 210nm; if you work out the East/West Departure distance at 45N it comes to 212nm-ish. You therefore have an approximate right angled isosceles triangle where the right angle is at 45N 15W. The Rhumb Line track from 10W to 15W along 45N would be 270; add the angle from the isosceles triangle (45degs); result approximately 315T.