R Markdown equation ` lefted as group` for Word

I know we can use $$ \begin{aligned} ....\end{aligned} $$ to realize math equation formulas alignment in Word using R Markdown. However, if I want to the entire equation (may be multiple lines) lefted as group in R Markdown for Word, how do I get that?

I also search here trying to use reference_docx: template.docx to get lefted as group (or lefted as group after 2em indent), but there seems to be only centered as group in Word!

My test example is given as follows:

---
output:
  word_document: 
    reference_docx: template.docx
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(message = FALSE, warning = FALSE)
```

blablablablablablablablablablablablablablablablablabla
$$
\begin{aligned}
a &= b+c \\
&= (b1+b2)+(c1+c2)
\end{aligned}
$$
hahahahahahahahahahahahahahahahahahahahahahahaha

I want to get (lefted as group after 2em indent)

blablablablablablablablablablablablablablablablablabla
    a = b+c
      = (b1+b2)+(c1+c2)
hahahahahahahahahahahahahahahahahahahahahahahaha

other than (centered as group)

blablablablablablablablablablablablablablablablablabla
                           a = b+c
                             = (b1+b2)+(c1+c2)
hahahahahahahahahahahahahahahahahahahahahahahaha