Standard form of origin-centred ellipse: x^2/a^2+y^2/b^2=1 where a = length of semi-major axis and b that of semi-minor axis. So plugging in a=5 and b=3: y^2/25+x^2/9=1. The x and y are interchanged because the elongation is with respect to the y axis so a and b are reversed. The equation can be written: 9y^2+25x^2=225.