I will do this in stages to enable you to learn to do it for yourself.
Aspect ratio = Span / Chord
If we multiply top and bottom by span we get
Aspect ratio = (Span x Span) / ( Chord x Span)
But Chord x Span is area so we have
Aspect ratio = Span squared / Area
Multiplying both sides by area gives us
Aspect ratio x Area = Span squared
Square rooting both sides gives us
Square root of ( Aspect ratio x Area) = Span
Inserting your numbers gives
Square root of (7.5 x 845) = Span = 79.6