Answer
about $1868$ Calories per day
Work Step by Step
$ R=73.3\sqrt[4]{M^{3}}\qquad$ ... substitute 75 for M
$ R=73.3\sqrt[4]{75^{3}}\qquad$ ... use calculator, $73.3\times 75^{\wedge}(3\div 4)$
$\approx 1868.0991$
.. about $1868$ Calories per day