Question 1110359
#5 says  "square b, negate the result, then add b"
#6 says "negate b, square the result, then add b"
#7 says "square a, multiply the result by 3"
—
There is no inconsistency in the notation.
—
While it is not an error to write {{{ 3(a)^2 }}},  it is also not something you will see in general.
—

Note that negation is the same as  multiplying a number by -1,  and so it occurs just like any other multiplication in the standard order of operations.   

———————————————————————————

To the student:  read my response carefully.   There is no inconsistency.   You are confusing 
(1) a minus sign appearing in an equation
 with 
(2) a variable taking on negative values
—
Sure, in  EVALUATING  {{{ 3a^2 }}} for a=-2,  one adds parenthesis {{{ 3(-2)^2 }}} because without them, it would change the meaning of the expression.   That's why I wrote out the meaning of each expression in words, and you should practice doing that.    There is no "negation" here, the expression is being evaluated for a value of 'a' that is less than zero.   Think of a number line, with zero at the middle. Numbers to the left of zero are negative, numbers to the right of zero are positive.   


——————————————————————————— 
Example 1
========
y = -a       <-——  means  "y equals negative a"   or equivalently,  "y equals -1 times 'a' "    

if a=-12,  then y = -(-12) = (-1)*(-12) = 12  
if a=64,  then y = -64                                         

——

The "-" sign in the equation y=-a in example 1  is similar to that of problem #5, while the VALUE -12 corresponds to the -2 they gave you to plug in to problem #7.
—

You are free to add parenthesis when it provides clarity.   You are REQUIRED to add parenthesis if the meaning of the expression would otherwise be changed without them (see example 2).

—————
Example 2
—————

 Many times on this site, students will ask  questions such as  " {{{ y = (3-x)/(2+5x) }}}, solve for x in terms of y"  but sometimes they don't use the formatting brackets and instead write   y = 3-x/2+5x.   To convey the proper expression, they should add parenthesis  y = (3-x)/(2+5x)
—
EDITED 2/25:  reworded very last sentence.