Question: the meaning of four consecutive even numbers is 15. the greatest of these numbers is a^1. the least of these numbers is .

First, a bit of housekeeping:

The meaning of four consecutive even numbers is 15.  Wouldn't that be "mean," not meaning?  Very different concepts!

The greatest of these numbers is a^1

"a^1" means "a to the first power.  There are no powers in this problem statement.  Perhaps you meant just "a" or "a_1" or a(1).

The least of these numbers is a^2.
No powers in this problem statement.  Perhaps you meant a_2 or a(2)

In this problem you have four numbers.  All are even, and there's a spacing of 2 units between each pair of numbers (consecutive even).

The mean, or arithmetic average, of these numbers is (a+b+c+d) / 4, where a, b, c and d represent the four consecutive even numbers.  Here this mean is 15.  The mean is most likely positioned between b anc c.

So here's what we have:  a+b+c+d
                                          = 15
                                                 4

This is equivalent to a+b+c+d = 60.

Since the numbers a, b, c and d are consecutive even integers, let's try this:

a + (a+2) + (a+4) + (a+6) = 60.    Then 4a+2+4+6=60, or 4a = 48, or a=12.

Then a=12, b=14, c=16 and d=18.  Note how (12+14+16+18) / 4 = 15, which is the given mean.

We could also type, "a(1)=12, a(2)=14, a(3) = 16, and a(4) = 18.