At the beginning of an environmental study, ad forest covered an area of1500km2.Since then, this...

At the beginning of an environmental study, ad forest covered an area of $1500k{m}^2$.Since then, this area has decreased by 4.25% each year.Let t be the number of years since the start of the study.Let y be the area that the forest covers in $k{m}^2$. 
Write an exponential function showing relationship between y and t.