What’s the missing side?

What properties allow transformation to be used as a problem solving tool

The center of the circumscribed circle lies on line segment and the longest side of the triangle is equal to the of the circle.

Amass weighing 16 pounds stretches a spring 8/3 feet. the mass is initially released from rest from a point 2 feet below the equilibrium position, and the subsequent motion takes place in a medium that offers a damping force that is numerically equal to 1/2 the instantaneous velocity. find the equation of motion if the mass is driven by an external force equal to f(t)=cos(2t).