If x is the amount she sold on Thursday, then on Friday she sold 2x+3.25=20.25. The question says $3.25 more than twice which means $3.25 added to 2x, 3.25+2x (which is the same as 2x+3.25) as an algebraic expression. So now we can solve the equation: 2x=20.25-3.25=17, so x=17/2=$8.50.
The amount she sold on Thursday was $8.50.