You need to set up 2 equations for your 2 unknowns.

First equation:

x+y=200 (x is the $3 ticket and y is the $2 ticket)

Second equation:

3x + 2y= 475 (the sum of all tickets sold is 475)

In the first equation solve for x:

x=200-y

Now plug in x in the second equation:

3(200-y) +2y = 475

600-3y+2y = 475

600 -y = 475

-y = -125

y=125

Now plug y into the first equation:

x=200-125

x=75

So there were 75 $3 tickets sold and 125 $2 tickets sold