Question

Find two numbers a and b whose sum a+b is -1 and whose difference a−b is 5.

I do not understand the question

Answer 1

a + b = -1
a - b = 5

a = -1 - b
Substitute:

-1 - b - b = 5
-2b = 6
b = -3

a - 3 = -1
a = 2

Answer 2

a = b + 5

a + b = -1

Substitute b+5 for a in the second equation and solve for b. Insert that value into the first equation to solve for a. Check by putting both values into the second equation.