So since you gave all of the equations of everyone's amount that they put in, it saves some work.
Let's add up everyone's contribution and then solve for x:
Then since we know the television costs $2,000, we can set this expression equal to that and simplify terms. Then we can solve for x:
So since Ronnie put in 'x', we can say that Ronnie paid $70.
Since Mark put in '2x + 500', we can plug in 70 for x to find that Mark paid $640.
And since Lisa put in '650 + 2x + 500', we can plug in 70 for x again to find that Lisa paid $1,290.