Natural convective heat transfer rate in an enclosure of a trapezoidal configuration is numerically investigated with a sinusoidally heated bottom wall. The enclosure is surrounded by a sinusoidally heated lower wall, the upper cold wall, and the right vertical and the inclined left surfaces are adiabatic. Galerkin's residual finite element procedure is implemented to resolve the governing equations. The characteristics of fluid flow and heat transfer are studied over a wide range of variables such as Rayleigh number, 10² ≤ Ra ≤ 10³, and height of cavity, H = 0.5. The values are furnished in the form of streamlines, temperature contours, and Nusselt numbers. It is noticed that the Nusselt number rises with the rises in Rayleigh numbers. Further, the overall heat transfer rate reduces with a decrease in the length of the cold wall. The power law and multiple regression correlations are presented between Rayleigh number, Nusselt number, and cooling length.