First expand the term in parenthesis by multiplying each term within the parenthesis by the term outside the parenthesis:
19a - color(red)(3)(a - 6) = 66
19a - (color(red)(3) * a) + (color(red)(3) * 6) = 66
19a - 3a + 18 = 66
Next, combine like terms:
(19 - 3)a + 18 = 66
16a + 18 = 66
Then, subtract color(red)(18) from each side of the equation to isolate the a term while keeping the equation balanced:
16a + 18 - color(red)(18) = 66 - color(red)(18)
16a + 0 = 48
16a = 48
Now, divide each side of the equation by color(red)(16) to solve for a while keeping the equation balanced:
(16a)/color(red)(16) = 48/color(red)(16)
(color(red)(cancel(color(black)(16)))a)/cancel(color(red)(16)) = 3
a = 3