To solve this using probability simply count the shapes that have at least 4 sides, and represent it as a fraction among all shapes in the spinner.
P(4 sides) = 5/6.
P(tails) = 1/2.
Both are independent events and as such must be multiplied with each other.
5/6 • 1/2 = 5/12,
This is the combined probability of both events occurring.