We have that,
Price of each cell phone plan = $x
Since, 3 friends pay extra $4 for the plan.
Thus, the amount paid by the three friends is $ 3(x+4).
Now, as the ramaining people do not pay the extra cost and there are 7-3 = 4 people remaining.
Thus, the amount paid by remaining people = $ 4x
Thus, the total cost of the plan will be $ [3(x+4) + 4x] = $ (7x+12).
Hence, the expression for the total cost of the seven plans is $ (7x+12).