Alan

Write the three equations as:

-22 = 16a - 4b + c.  (1)

2 = a - b + c.           (2)

-1 = 4a + 2b + c.     (3)

(3) - (2):  3a + 3b = -3.  (4)

(1) - (2):  15a - 3b = -24  (5)

(4) + (5):  18a = -27.   Divide by 9:     2a = -3.     a = -3/2

Substitute for a in (4) or (5) to find b, then substitute both into any of (1), (2) or (3) to find c.

You don't need to find the explicit form of f(x)¹⁶ here.  See the following:

This graph should help

If the numerator is meant to be 2*(n!), the ratio of the n+1th term to the nth term becomes (n+1)/4. This is greater than 1 for all n>3, so the sum still diverges.  However, I think this interpretation of 2n! is unlikely.

numerator of nth term   (2n)!

 numerator of n+1th term (2*(n+1))!  =  (2*n + 2*1) ! = (2n + 2)! = (2n+2)*(2n+1)*(2n)!

I think this has been asked and answered previously somewhere, but I couldn't find where, so:

As follows:

"The variable x varies directly as the cube of y, and y varies directly as the square root of z. If x equals 1 when z equals 4, what is the value of z when x equals 27?"

As follows:

You're welcome!