Let's simplify the equation step by step:

90 - 3a - (a + 8) = 7

First, distribute the negative sign through the parentheses:

90 - 3a - a - 8 = 7

Combine like terms:

90 - 4a - 8 = 7

Now, combine the constants:

82 - 4a = 7

Next, subtract 82 from both sides of the equation:

-4a = -75

Finally, divide by -4 to solve for 'a':

a = 18.75

So, the solution to the equation is a = 18.75.