Rose went to a stationary shop. she purchased 2 packs of red pens; 4 packs of black pens; and 3 packs of blue pens. the cost of each pack of pens was $2.50.which expression shows the total amount rose spent on the pens?

Well since you didn't provide the choices, I'll have to make some assumptions... we'll call the red pens R, the black pens B and the Blue pens x. If total cost = T then We can say...
T=R(2.50)+B(2.50)+x(2.50)

It could also be shown as

T = R + B + x(2.50)

Another way that we could have approached it since there is no difference in price between colors is just say each pack of pens (no matter the color) = P... so...

T = P (2.50)