fundamental astronomy - Finding Mass of Star with only Luminosity

Sounds like homework (correct me if that assumption is wrong, as you may want a somewhat different answer then.), so here is a hint:

Clean up the equation first, to something that looks more like an equation for mass:

$$frac{L}{L_{sun}} = left(frac{M}{M_{sun}}right)^{3.5}$$

$$left(frac{L}{L_{sun}}right)^{frac{1}{3.5}} = frac{M}{M_{sun}}$$