Ms. Richardson bought six 2-gallon containers of iced tea. To find the total amount of iced tea she had, we can multiply the number of containers by the amount of iced tea in each container:
\[ 6 \text{ containers} \times 2 \text{ gallons/container} = 12 \text{ gallons} \]
Her guests drank 7.75 gallons of iced tea. To find out how much iced tea she has left, we subtract the amount consumed from the total amount:
\[ 12 \text{ gallons} - 7.75 \text{ gallons} = 4.25 \text{ gallons} \]
Thus, Ms. Richardson has 4.25 gallons of iced tea left.