Light is travelling towards the right.

n1/p + n2/q = (n2 - n1)/R

The index in the object space is 1.5 in this case.
The index in the image space is 1 in this case.

1.5/3 + 1/q = (1 - 1.5)/(-2)

q = -4 cm

A negative value for q means that the distance from the curved surface of the paperweight to the image is against the flow of light.  That is, the image is 4 cm to the left of the curved surface of the paperweight.

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