Which correctly gives the expression using addition? –17.9 – 4.2
a. –17.9+4.2
b. 17.9+4.2
c. –17.9+(–4.2)
d. 17.9+(–4.2)

c. –17.9+(–4.2)

The General rule for adding and subtracting numbers 
1. Two integers with the same signs
Once 2 integers has the same sign, then just add the numbers.
For example 
1. 1+1 = 2 
2. 2 + 5 = 7   

2. Two integers with different signs
When 2 integers has different sign, then find the difference
For example
1. 1-1 =0 
2. 2 – 5 = -3 

3. Two integers that vary in sign
When 2 integers vary in sign, then it will depend who which number carries the largest value
For example 
1.   -3 + 2 = -1 
2.   2 – 1 = 1