PE(MD)(AS).

The P stands for parentheses,
The E stands for Exponents.
The (MD) is for multiplication or division, whichever comes first going left
to right.
The (AS) is for addition or subtraction, whichever comes first going left to
right.

136-4[6^2-(8-4)]

The "P" of PE(MD)(AS) says "Do the innermost parentheses first. There are
parentheses () within parentheses [].  We start with the innermost
parentheses, (8-4)
  
PE(MD)(AS) starts over. There is no parentheses inside that parentheses, so
we go to the E for exponents.  There are no exponents inside (8-4), so we
go to the (MD) for multiplication/division.  There is neither of those
inside (8-4).  So we go to the (AS) for addition/subtraction.  There is indeed
a subtraction, 8-4, so we replace the (8-4) by (4), and since there are no
operations inside that parentheses, we can drop them and just replace (8-4)
by 4 and copy everything else over:

136-4[6^2-4]

PE(MD)(AS) starts over each time. "Do the innermost parentheses first, which is
[6^2-4].  There are no parentheses inside the parentheses [], so we go to
the next letter of PE(MD)(AS) which is "E" for exponents.  There is an exponent
to do there. So [6^2-4] becomes [36-4] so we replace [6^2-4] by [36-4] and
copy everything else over:

136-4[36-4]

PE(MD)(AS) starts over. "Do the innermost parentheses first, which is [36-4].
There are no parentheses for "P" inside the parentheses [].  So we go to
"E".  There are no exponents for "E", so we go to (MD).  There are no
multiplications or divisions, so we go to (AS).  There is a subtraction,
so we subtract 36-4 and get [32].  So we replace [36-4] by [32]. We must
keep the parentheses around 32 this time because if we were to put 32 next
to 4 it would look like 432. So we put [32] in place of [36-4]:

136-4[32]

The "P" of PE(MD)(AS) says do the parentheses first [32].  But there is
nothing to do inside the parentheses because 32 is just a number.  There are
no exponents so we go to (MD) and we find there is a multiplication to do,
4[32].
So we multiply 4 by 32 and get 128. So we replace 4[32] by 128 and copy
everything else over.

136-128

There is no P, E, (MD), so we go to (AS). There is a subtraction, so we do
that and the final answer is

8.

Edwin