Excuse my saying this, but it's a little weird for your dad to tell you that you're doing something wrong and then walk away and leave you hanging, without helping you do it right.
And there's another weird thing here: The equation you squeezed in there in the blank line is EXACTLY CORRECT. That's exactly the answer to the next line, where it ways "Define a variable and then write ... an equation ...". You've already got that, and you're on your way.
In your equation, 'x' is the price of the second painting. When you solve your equation, you'll know that, and you'll be able to find the price of the first one. Then you'll immediately know which one was the most expensive.
The important thing is: You've already finished the toughest part ! The rest is just some arithmetic. Great work !
