Answer:
a) System of equations will be:

b) Number of adult tickets sold = 263
Number of students tickets sold = 237
Explanation:
Let:
Number of adult tickets sold = x
Number of students tickets sold = y
a)
As Marc sold total 500 tickets, the expression will be:

Student tickets cost $2 and adult tickets cost $5. Marc's sales totalled $1,789.
The expression will be:

So, system of equations will be:

b)
Solve the system to find value of x and y
Let:

Multiply equation 1 by 2 and subtract

We get value of x = 263
Now finding value of y by putting value of x in eq(1)

We get value of y = 237
Number of adult tickets sold = x = 263
Number of students tickets sold = y = 237