Let the two side lengths of the tetrahedron be a and b. According to Hint 2, you are supposed to express \frac{\mathrm d a}{\mathrm dt} and \frac{\mathrm d b}{\mathrm dt} in terms of a and b. By dividing these two expressions, you can obtain an expression for \frac{\mathrm d a}{\mathrm db}. Upon solving this differential equation, you can express a in terms of b (or the other way around – whichever is easier). Now, with this expression, you can eliminate one of the variables (a or b) in either the expression for \frac{\mathrm d a}{\mathrm dt} or in the expression for \frac{\mathrm d b}{\mathrm dt}. As a result, you will obtain a separable differential equation that can be integrated to find the evolution time t.
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