Let x be number of days.
and y be number of people attending the carnival.
Since it is increasing linearly, so the problem can be described using line equation. y-y1 = m(x-x1)
where m = (y2 - y1)/(x2-x1) = (575-425)/(3-1) = 75
Therefore using line equation we know,
y - 425 = 75(x-1) ----------------------(1)
Now putting the value of x=6 in eq. 1 we get:
y - 425 = 75(6-1)
y = 75*5 +425
y = 800 peoples
So on 6th day, 800 peoples will attain the carnival