Well, since you have the value of b, the first thing you want to do is substitute that value in for b.
5a - 10(3) = 45
10*3 = 30
5a - 30 = 45
Now, to get the variable by itself (a) you have to + 30 because you have to reverse the subtraction.
5a = 75
Now, you divide by 5 to finish getting a by itself.
a = 15
Hope this helps!
