Sorry, the way this is written is a little confusing, but I’m assuming you mean the equation is x^2 + 18x + 12? Sorry if I’m mistaken. If this happens to be the case, then the a value would be 1, the b value would be 18, and the c value would be 12. Again I’m assuming this is in the ax^2 + bx + c formula. I hope this helps, sorry if I’m wrong :/
