The decay of mercury-178 (17880Hg) by positron emission involves the emission of a positron (a positively charged electron). Here's the balanced nuclear equation for this decay:
17880Hg -> 17779Au + 0+1e
In this equation:
17880Hg represents the mercury-178 nucleus.
17779Au represents the gold-177 nucleus, which is the result of the decay.
0+1e represents a positron (e+), which is emitted in the process.