You are on the right track in terms of the expressions you created.
Ronnie: x Lisa: 2x+500+650 Mark:2x+500
To solve for how much everyone contributed, we have to solve for x, then use the rest of the expressions to solve for each person. Since we know the TV cost $2000, we can add all the expressions together and solve.
2000=x+2x+500+650+2x+500
2000=5x+1650
5x=2000-1650
5x=350
x=70
Now that we know x=70, we can use our expressions to solve for what each person contributed. Ronnie is easy because we already know he contributed $70.
Lisa: 2x+500+650 L:2(70)+1150 L:140+1150 Lisa: $1290
Mark: 2x+500 M:2(70)+500 M:140+500 Mark: $640