I'm pretty new to R and not a statistics expert.

I'd be very grateful if someone could help me with the following question:

I'm using the following R code for the calculation of a varying intercept, varying slopes LMER model:

> Model.2004 <- lmer(var1 ~ var2 + var3 + var4 + var5 + var6 + var7 + var8 + var9 + var10 + var11 + var12 + (1 + var9 + var10 + var11 + var12 | district)

I would like to write that code in formal mathematical notation. Thanks for helping me out!