We start by using the distributive property to expand the terms

and

. When we do, we obtain

and

.
Putting those back into our equation, we have:

Next, we combine all of the constants on either side:

Finally, we want to get the variable a by itself on one side of the equation, so we perform the following steps to isolate it:

(Add 20 to both sides)

(Subtract 3a from both sides)

(Divide both sides by 2 to isolate a)
So, we obtain our answer of

.