You already have that the RHS (right-hand side) of your expression, abc/(4R) is one way to calculate the area of the triangle.

Thus you can focus on the LHS. Here's a hint for there. The (s-a)(s-b)(s-c) reminds us of something we recognize from Heron's formula, except Heron's formula has an extra s in front. What happens if we multiply the numerator and denominator of that side by s?