Complete the square for -16x²+144x-80 show all work

Since this does not factor you have to use the quadratic equation:
x= ( −b±√ b 2 −4ac )/2a
In this case your b=144 a=-16 and c=-80.  Make sure you keep the negative sign from the minus with the 80.  Then you plug in and do the math.
(-144±√(144)²-4(-16)(-80))/2(-16)
(-144±√20736-5120)/-32
(-144±√15616)/-32
(-144±16√61)/-32
So your two answers are:
x=(
x=

Hope that helps.  Let me know if any of that does not make sense.