Great

Thank you, I rewrote: When my friend suggested I take a cooking course, I replied that it would be very useful. I will take you up on that offer.

You can say, “…cooking course, I replied that it would be very useful.” or “…I thanked him and said it would be useful.”

When you say, “When my friend suggested…” you need to follow it with something that depends on the suggestion. So, “When my friend suggested I take the course, I replied that it would be helpful.”

But simply, “it would be helpful” is true whether your friend suggests it or not.