in the first on a can only be 18 but in the second a can be 18 or greater!

wgs

It has given you a - 3 = 15 and and a - 3 is greater than or equal to 15.
Think about it. The first equation has only one solution. There is only one answer for a.
The inequality however, has that solution AND a bunch of other numbers greater than it.
So they are similar I'm that they share a solution, but different because the inequality has many solutions, but the equation has just one.