Writing linear functions word problems

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A young sumo wrestler decided to go on a special high-protein diet to gain weight rapidly. He started at 9090 kilograms and gained weight at a constant rate. After 88 months, he weighed 138138 kilograms.
Let W(t)W(t) denote the sumo wrestler's weight WW (measured in kilograms) as a function of time tt (measured in months).
Write the function's formula.
W(t)=W(t)=