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

9000 - 3x = 1200

Next, we can isolate the variable by subtracting 9000 from both sides:

-3x = 1200 - 9000

-3x = -7800

Finally, we can solve for x by dividing both sides of the equation by -3:

x = -7800 / -3

x = 2600