[mistine]

Jane collected some rocks on Monday. On Tuesday, she collected 3 more than on Monday. On Wednesday, she collected 6 more than on Tuesday. She collected 12 on Wednesday. How many rocks did she collect on Monday?

[fredg]

I will not give you the answer, but will help you, I hope, get started with doing the problem.
I did teach high school math for 13 yrs, before changing careers.
First,
Let x = the number of rocks collected on Monday.
Then, x + 3 = number of rocks collected on Tuesday.

Now, you set up the expression for Wednesday.
When you get that, set that expression = 12.
When you solve it, the x will be the number collected on Monday.
Good luck.