Because t is the number of weeks, replace t with 6 in the equation and solve for s.
s = -0.25(6) +4
s = -1.5 +4
s = 2.5
Now replace t with multiples of 6, use 12 and 18:
s =-.25(12) +4 = 1
s-0.25(18) +4 = -0.5
now fins the difference between the time frames:
2.5 - 1 = 1.5
1 - 0.5 = 1.5
The ice melts 1.5 meters every 6 weeks.