I'm guessing at what you want. I think you want to know the number, but you could also just want the equation.
Step One
Start with "the sum of a number and - 3." That looks like this.
x + - 3 = x - 3
Step Two
"Is multiplied by - 5"
- 5 (x - 3)
Step Three
"... the result is 35"
-5(x - 3) = 35
Solve
You can remove the brackets or divide by - 5
-5x - 15 = 35 Add 15 to both sides
- 5x = 35 + 15
-5x = 50 Divide by - 5
x = 50/-5
x = - 10 <<<< Answer