A fruit and vegetable retailer buys melons from market gardener A. The mass in grams of melons from market gardener A is modelled by a random variable $M_\mathrm{A}$ that follows a uniform distribution on the interval $[850; x]$, where $x$ is a real number greater than 1200. Melons are described as ``compliant'' if their mass is between 900 g and 1200 g.\\
The retailer observes that $75\%$ of melons from market gardener A are compliant. Determine $x$.