Here's what you need to realize:
a + a + a is equal to 3 * a.
Now, if we look back at the equation:
3*a + 5 = 20
Both sides of the equation are equal, so if we subtract 5 from both sides, they will still be equal.
3*a + 5 -5 = 20 -5
The +5 on the left side of the equation is gone, and the right side of the equation is now 15.
3*a = 15
Divide both sides by 3 to find the value of a.
3*a / 3 = 15 / 3
a = 5