The sum of twice a number and 15 is - 42

2 (number) + 15=-42

2 (number) + 15-15=-42-15

2 (number) = - 57

2 (number) : 2=-57:2

number=-28.5

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