Conceptually, you are asking for the additive inverse of 8. By definition, in fact, the additive inverse of a number
is a number
such that 
You obtain the additive inverse of a number by switching its sign, so the additive inverse of 8 is -8. In fact, you have

What does coast mean in this question I would try to answer it
Answer:
31 scientific calculators
54 graphing calculators
Step-by-step explanation:
X = number of $15 calculators
Y = number of $67 calculators
X+Y = 85 then X = 85-Y
15X+67Y = 4083
substitute for X
15(85-Y)+67Y = 4083
1275 - 15Y + 67Y = 4083
reduce
52Y = 2808
Y = 54
X = 31