Run time error help

[4907]

I get this error when I test my code and after debugging this line gets highlighted. I've looked up what the error means but can't seem to figure out how to fix it.

Screen Shot 2016-09-16 at 7.34.34 PM.jpgScreen Shot 2016-09-16 at 7.34.25 PM.jpg
Thanks!

[mana]

Write "Option Explicit"

Dim x1 as double
Dim x2 as double
Dim xr as double
Dim es as double
Dim n as long

[4907]

Now it's saying "Compile error:Variable not defined" and highlighting the Sub secant() line

[mana]

declare all variables

Dim x1 as double
Dim x2 as double
Dim xr as double
Dim es as double
Dim n as long

[mana]

function procedure

private function fx(x as double) as double
	fx=(-x^2)+x+0.75
end function