First, let's distribute the 12 to both terms inside the parentheses:

12x + 36 - 5x - 21 = 0.5

Next, let's combine like terms:

7x + 15 = 0.5

Now, let's isolate the variable by subtracting 15 from both sides:

7x = -14.5

Finally, divide by 7 to solve for x:

x = -2.07142857143