Hey there! 
Since we don't have values for b, d, and r, we can't get a specific number for the value of a, if that makes sense. It'll be represented by these variables. 
To solve, we need to make sure the equation looks like a = ? . 
Subtract b from each side of the equation. 
<u>a = dr - b </u>
Now we have solved for a. 
Hope this helps!