A young sumo wrestler decided to go on a special high-protein diet to gain weight rapidly. He started at $90$ kilograms and gained weight at a constant rate. After $8$ months, he weighed $138$ kilograms. Let $W(t)$ denote the sumo wrestler's weight $W$ (measured in kilograms) as a function of time $t$ (measured in months). **Write the function's formula.** $W(t)=$ [[☃ expression 1]]