Q show curve y=cosh2x -10coshx+13 has three stat points and find y co-ord of stat points,,,

i get (0,4) (-ln(6.25+(5.25)^0.5,-6.19) (ln(6.25+(5.25)^0.5,6.19)

im using calc fx-9750 PLUS and the *** thing when i trace the stat points gives (0,4) and (-1.6,-0.5) and (1.6,0.5) so i concluded i must be wrong,,,,,,,does n e one know where ive gone wrong? previously useddouble angle cosh2x=2cosh^2x-1 then for y co-ords both expansion using cosh 2x=0.5(e^x etc and bunging in calc gave me 6.19)

can some1 pls help!