Diana bought a car for $5000 and plans to sell it to make a 20% profit. what should she sell it for to make the profit she desires?

You need to know 20% of $5000.  "Of" means "times".  

$5000 x 0.20 (which is 20% written as a decimal)

=$1000 profit she wants to make

$5000 (original price) + $1000 (profit) = $6000 (sale price)