You need one more fact either an angle or a side in order to be able to answer. I take it you have neither. But there must be something more you can tell us from the question. List the question exactly as it is asked with a diagram if necessary. I'll follow and use either comments or I'll edit.
The direct answer to your question is that you can't.
The bicycle costs $30 and Hugo finishes paying after 15 weeks. When he has paid for the bike, y=0 because he owes nothing. So -10=-2x+20, 2x=30, x=15. And the cost of the bike is 2×15=$30.
