The first two equations which are same see below

The question here appears to be in choosing the equation that matches the situation described, and capturing the order in which it is described. The commas suggest grouping as follows:

"He says the sum of the number and 8, multiplied 3/4, is equal to -12

Well the sum of the number and 8 describes a quantity (x + 8)

The first comma suggests grouping, the second comma would imply that the grouping is multiplied by 3/4

This makes (x + 8)3/4 or

3/4(x + 8) = -12 a choice

By the way, if you distribute the 3/4 through the parentheses you get

3/4x + 6 = -12 so of course

3/4(x + 8) = -12

and

3/4x + 6 = -12

Represent the same equation and give the same answer

3/4(x + 8) = -12

3/4x + 6 = -12

Are both valid choices for Glopal's number.

However, a couple of questions could arise regarding the placement of the commas

in reading one would stop at a comma suggesting primarily the equations above.

On the other hand, one could wonder if the comma means stop multiply 8 by 3/4 first then include it in the sum suggesting that the comma is restrictive making.

x + 6 = -12 as an option

By the way this is what you would get without the commas

The sum of the number and 8 times 3/4 equals -12

x + 8*3/4 = -12

Well we should also consider what the question as described does not say

It does not say the sum of the number times 3/4 and 8 is equal to -12 or this 3/4x + 8 = -12

It does not say the sum of 3/4 times the number and 8 is equal to -12 or again 3/4x + 8 = -12

A suggestion here would also be to consult some English references about the commas, like your English teacher.

Give it a try, check on my interpretation of the commas and send me a message if you have any questions.