Answer : $1700
From the given information we make constraints
Let x be the number of large buses
and y be the number of small buses
C be the total cost for the trip
Cost C = 300 * number of large buses + 200 * number of small buses
So C = 300 x + 200 y
Constraints are :
at least 360 students, So 
at most 7 buses , So x + y < = 7
Number of large buses x>=0
Number of large buses y > =0
Now we graph the constraints and find all the endpoints
Graph is attached below
End points are (3,4) , (6,0) and (7,0)
Now we find cost at each end points
C = 300 x + 200 y
(3,4) ----> 300(3) + 200(4) = 1700
(6,0) ----> 300(6) + 200(0) = 1800
(7,0) ----> 300(7) + 200(0) = 2100
The minimum cost is 1700