Alan

Perhaps this graph will help:

Multiply the first equation by 2:

2y - 6x = 2a      (1)

Subtract x and b from both sides of the second equation and rewrite as:

2y + 2x = -b      (2)

Subtract (1) from (2)

8x = -b - 2a   (3)

Multiply (2) by 3:

6y + 6x = -3b     (4)

Add (1) and (4)

8y = 2a - 3b    (5)

You can now divide (3) by 8 to get x and (5) by 8 to get y

As follows:

Like so:

Missing data in the question!

A short computer program gives the 100'th term as 792070839848372253127, so the remainder on dividing by 20 is 7.

(An internet search might well produce a closed form expression for the n'th term.)

See https://web2.0calc.com/questions/help-pls_85792#r1

See the following:

You now hve four independent equations in four unknowns.  Can you take it from here?

When f(x) multiplies g(x) there will be a term 2x⁸.  No matter what size the constant b is, it won't affect the x⁸ term.  Hence the degree of the resulting polynomial will be 8.

Total = 9*a + 3*c

Now, just plug in 229 for a, and 385 for c.