A business uses straight-line depreciation to determine the value y of a piece of machinery over a 10-year period. Suppose the original value (when t=0) is equal to $42,000 and its value is reduced by $4200 each year. Write the linear equation that models the value y of this machinery at the end of year t. The linear equation models the value y of the machinery at the end of year t. (Type an equation. Do not factor.)