Question

why does 5k+3k^3+7k+9k^3 = 12k^3

Answer 1

It doesn't...it should be = 12k^3 + 12k

Answer 2

If you simplify things, you get
12k^3 + 12k = 12k^3

You can then subtract 12k^3 from both sides to get:

12k = 0

Therefore, k=0

So, really your equation is correct because when you evaluate the left side, you end up with 0 and when you evaluate the right side, you get 0 and

0 = 0