How would I work out & produce a written mathematical equation to the project below?

The project is estimated to cost £18 million in today's prices.
The start of the project is 2018, construction costs are rising at 4%, while long term investment gains 6%.
How would I produce a mathematical equation to advise on the 2018 build cost and the level of investment needed to secure the project?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).