-15
Step-by-step explanation
Step 1
let x represent the number
now, traslate into math terms
a)six more than a number is negative nine

hence, we got the equation

Step 2
now, solve the equation

we need isolate x without affect the expression, The subtraction property of equality tells us that if we subtract from one side of an equation, we also must subtract from the other side of the equation to keep the equation the same
hence, subtract 6 in both sides

therefore, the number is -15
I hope this helps you