Easy# Finding the Radius of a Ring of Charge

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Figure 1 depicts a ring of charge of radius $R$ whose center lies a distance $z$ from a field point $P$.

If the linear charge density (charge per unit length) of the ring of charge is equal to $\lambda = 1.30 \: \rm\frac{nC}{m}$, we can plot the electric potential at point $P$ as a function of the displacement $z$, for some particular value of $R$, shown in Figure 2.

What is the radius $R$?