For Felicia to have at least 120 buttons, k will have to be greater than or equal to 8.
Here's how to find that.
I found it's easiest to create an X,Y table (or K,L table as shown in the image below). The X (K) value starts at 0 (the start of collection) and continues to go up by by 1 to account for every year she collects buttons, while the Y (L) value starts at 29 (starting value of buttons) and continues to go up by 12 each year.
By creating this X,Y table, you'll find that 8 years is when she will have finally collected at least 120 buttons. Below is an image of the graph to help elaborate further.
I hope this helps!