r – exponential decay fitting line and equation in the plot using ggplot2

I have a dataset www.dropbox.com/s/qemmepoq105rth2/sample_data_trasport.csv?dl=0

using code

 shapes <- c(5, 16, 18, 3, 15, 4, 6, 8, 17, 13, 2)

trasport_time %>% 
filter(PM2.5 > 25) %>% 
ggplot(aes(x = Transport_time_hrs, y = mac_405, shape=(Fire_Name),
           color=(day))) +
geom_point( alpha=1, size = 4)+  scale_shape_manual(values = shapes)+
xlim(0,25)+ 
ylim(0,1.5) + #geom_smooth(aes(group = 1),method=nls,  se=F)+
geom_smooth(aes(group = 1), method = "nls", formula = y ~ a * x^b, 
            se = FALSE, method.args = list(start = c(a = 1, b = 1))) +
theme(legend.text=element_text(size=12)) + 
theme(axis.title = element_text(face="plain",size=14,color="black"),
      axis.text=element_text(size=12,face="plain", color="black"),
      plot.title = element_text(size=15)) +
ylab(bquote("MAC"[(405*nm)]~ '('*m^2*g^-1*')')) +
xlab(bquote('Transport Time ('*hrs*')')) +  
labs(title = "")+ labs(shape="Fire", color="Day")

From this code, I am, able to draw the fitting line, but I am not able to add the fitting equation for example: y(t)= y(e)−kt (k = half-life

I want to add something like (enter image description here)

enter image description here

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