ExpressionEngine PHP Error: Non-static method should not be called statically

###Problem

```A PHP Error was encountered Severity: 8192 Message: Non-static method Mx_calc::usage() should not be called statically, assuming $this from incompatible context Filename: mx_calc/pi.mx_calc.php Line Number: 20

```A PHP Error was encountered
Severity: Warning
Message: Cannot modify header information - headers already sent by (output started at PATH/system/codeigniter/system/core/Exceptions.php:170)
Filename: core/Common.php
Line Number: 479

###Solution

In this case, we need to modify Mx_calc, as stated in the error message:

Go to /system/expressionengine/third_party/mx_calc/pi.mx_calc.php
Look for function usage() around line 61
Replace it with public static function usage()

####Before

```function usage() { ob_start(); ?>

####After

```public static function usage()
{
ob_start();
?>