Nobody can guarantee a position in organic search results, and anybody who does is selling snake oil. What can be said honestly is that the work is measurable: you will know what was done, what moved, and what it cost, because we produce a plain English report each month explaining what changed and why.
The other honest thing to say is that this takes time. If you need business tomorrow, seriously consider pay-per-click advertising. In the meantime, investing in SEO will gradually ease your reliance on advertising. A site that has been neglected for two years does not recover in a fortnight, and a new site has to earn its place against sites that have been there for a decade.