-3x+9x

or you can use ower lovely calculator

go on google and serch calcuator and tip in (-3)+9

9 plus -3 is equal to 6

(9-3)

then add the variable onto the six.

(6 times x)...(6x)

Let me explain in detail, so you can apply this to other problems of the same kind.

When the variable is the same, then you can add the coefficients (numbers before the variables) like normal.

Since the variable (x) in -3x + 9x is the same you can just add the coefficents together: -3 + 9.

When you add a negative number to a positive number, it's the same thing as subtracting that negative.

So: -3 + 9 = 9 -3. Since 9 - 3 = 6, our answer is 6x.