Question 43850
ax+by=3     ..1
ax-by=10    ..2
adding equations (..1) and (..2) together gives:
2ax=13          ..3
ax=13/2=6.5     ..4
substitute equation (..4) into (..1) to give:
6.5+by=3
by=-3.5


I hope this helps.

P.S. I am trying to start my own mathematics homework help website.  I would be extremely grateful if you would send me your feedback on my help to adam.chapman@student.manchester.ac.uk.