まず、f(k-1)の項を反対側に移動させる必要があるが、これは両辺から引くことでできる。これによって、次の式が得られる：

f(k) - f(k-1) = 3k

ここで、両辺を3で割って、右辺のk項を分離することができる。これで次の式が得られる：

f(k) - f(k-1) = 3k

3(f(k) - f(k-1)) = 3k

f(k) - f(k-1) = k

最後に、両辺にf(k-1)を足すと最終的な答えになる：

f(k) = k + f(k-1)
Sure, I can help you solve this equation for k. The first step is to move the f(k-1) term to the other side, which can be done by subtracting it from both sides. This gives us the following equation:

f(k) - f(k-1) = 3k

Now, we can divide both sides by 3 to isolate the k term on the right side. This gives us the following equation:

f(k) - f(k-1) = 3k

3(f(k) - f(k-1)) = 3k

f(k) - f(k-1) = k

Finally, we can add f(k-1) to both sides to get the final answer:

f(k) = k + f(k-1)