A lake near the Arctic Circle is covered by a thick sheet of ice during the cold winter months. When spring arrives, the warm air gradually melts the ice, causing its thickness to decrease at a rate of $0.2$ meters per week. After $7$ weeks, the sheet is only $2.4$ meters thick. Let $S(t)$ denote the ice sheet's thickness $S$ (measured in meters) as a function of time $t$ (measured in weeks). **Write the function's formula.** $S(t)=$ [[☃ expression 1]]