In any case, if it's of any help, the Earth's orbit lies in a plane 7.2 degrees inclined to the equatorial plane of the Sun. Since the Sun differentially rotates depending on the latitude you measure, the given definition of the equator is the line tracing the circumference of the Sun where the rotation is equal to the average time taken for the Sun to rotate i.e. 25.4 days.
Here's something which might help, but it will be more to your benefit as it will most likely be a little too advanced for a Yr12 student to follow. Although, if they're good at maths, give them a go at it
