7 plus the quantity of 8 minus 6

The only part here that could really be confusing would be the phrase "the quantity of."
What this effectively means is that the next portion is placed in parenthesis and thus evaluated before the preceding portion. This can be important when order-of-operations is at issue (though it isn't here).
Other examples:
4 times the quantity 3 plus 2
= 4 x (3 + 2) = 4 x 5 = 20

while
4 times 3 plus 2
=4×3+2=15+2=17

Where is the