Question

The sum of twice a number and 15 is -42

Answer 1

2(number)+15=-42
2(number)+15-15=-42-15
2(number)=-57
2(number)÷2=-57÷2
number=-28.5

Answer 2

assume that the number is n.
sum of twice the number and 15 means that you will first multiply the number by 2 then add 15.
the equation us:
2n + 15 = -42
2n = -42-15
2n = -57
n = -28.5