Let #y^2-Py-10# be #f(y)#.
Since #y-2# is a factor of #f(y)#, equate #y-2# to zero and make #y# the subject of the formula
# y-2=0#
# y=2#
Equate #f(y)# to zero and substitute for #y=2#
# y^2-Py-10=0#
# 2^2-2P-10=0#
# 4-10-2P=0#
# -6-2P=0#
Make #P# the subject of the formula
# -6-2P=0#
# -2P=6#
# P=6/(-2)#
# P=-3#