To solve the equation (2600+6400)-3x=1200, we can simplify the left side of the equation first:

(2600+6400)-3x = 9000 - 3x

Now, we can set this equal to 1200:

9000 - 3x = 1200

To isolate the variable, we can subtract 9000 from both sides:

9000 - 9000 - 3x = 1200 - 9000

Simplifying further:

-3x = -7800

To solve for x, we divide both sides by -3:

(-3x)/-3 = (-7800)/-3

Simplifying:

x = 2600