To evaluate the expression, we use the formula for permutations:
nPr = n! / (n - r)!
There are 7 bands volunteering to perform, but only 4 can play. So, n = 7 and r = 4.
Using the formula:
7P4 = 7! / (7 - 4)!
7P4 = 7! / 3!
7P4 = (7 x 6 x 5 x 4 x 3 x 2 x 1) / (3 x 2 x 1)
7P4 = 5040 / 6
7P4 = 840
Therefore, there are 840 possible lineups for the benefit concert with 7 bands and 4 performing slots.