  Large-scale spiral arms have been revealed in scattered light images of a few
protoplanetary disks. Theoretical models suggest that such arms may be driven
by and co-rotate with giant planets, which has called for remarkable
observational efforts to look for them. By examining the rotation of the spiral
arms for the MWC 758 system over a 10-yr timescale, we are able to provide
dynamical constraints on the locations of their perturbers. We present
reprocessed Hubble Space Telescope (HST)/NICMOS F110W observations of the
target in 2005, and the new Keck/NIRC2 $L'$-band observations in 2017.
MWC~758's two well-known spiral arms are revealed in the NICMOS archive at the
earliest observational epoch. With additional Very Large Telescope (VLT)/SPHERE
data, our joint analysis leads to a pattern speed of
${0.6^\circ}^{+3.3^\circ}_{-0.6^\circ}\, \mathrm{yr}^{-1}$ at $3\sigma$ for the
two major spiral arms. If the two arms are induced by a perturber on a
near-circular orbit, its best fit orbit is at $89$ au ($0.59"$), with a
$3\sigma$ lower limit of 30 au ($0.20"$). This finding is consistent with the
simulation prediction of the location of an arm-driving planet for the two
major arms in the system.
