Rewrite y = 5(10^(−3x)) in the form

y = Ae^(mx). (i.e., determine A and m)

y/5 = 10^(-3x)

ln (y/5) = ln (10^(-3x)) = -3x(ln10)

y/5 = e^(-3ln10 x)
y = 5 e^(-3ln10 x)

I checked using x= .21 in both equations