Your processor does not support VT-x.
https://ark.intel.com/products/33915/Intel-Core2-Duo-Processor-T5750-2M-Cache-2_00-GHz-667-MHz-FSB Intel® Core™2 Duo Processor T5750 (2M Cache, 2.00 GHz, 667 MHz FSB) Product Specifications
The link takes you to the specifications of your processor.
Scroll down to Advanced Technologies. You will see that VT-x is simply not available on your processor.
How can you use it? You cannot.