The way we approach a problem like this is by using a technique called "collecting like terms".
This means that we want to put all of the values with an "x" on one side and all of the normal numbers on the other. For this particular problem we can do this as follows:
5x - 6 = x + 12
Now move the (- 6) to the right side of the equation by adding 6 to both sides:
=> 5x = x + 12 + 6
Now move the (x) to the left side of the equation by subtracting (x) from both sides:
=> 5x - x = 12 + 6
Now perform the simple arithmetic on both sides:
=> 4x = 18
Now remove the 4 from in front of the (x) by multiplying the whole equation by (1/4)
=> x = 18/4
Therefore to make your open sentence true, x must equal 18/4, or to simplify, 9/2, or as a decimal, 4.5. I hope this helped, and remember to try and understand not just the answer, but the maths involved in getting to the answer too :))
