I tried many equations (Stefan-Boltzmann constant) that gave me a temperature of 249.769 Kelvin (-23.38°C) assuming no atmosphere and 297 Kelvin (24°C) with perfectly absorbing one, but my planet is neither, and SpaceEngine is hopeless (Although it may be right) it gave me a temperature with greenhouse effect of 267.6 Kelvin (-5.5°C (The greenhouse effect is 16.373° Celsius/Kelvin.)).
So how can I calculate it?
My planet is orbiting a K type Star/Orange dwarf and doesn't have life on it.
Planet:(Unit is Earth = 1) Mass: 1.8 | Radius: 1.2 | Atmosphere: 88% N2 / 9% CO2 / 1.5% Argon / 1.5 other gasses. with Presure at sea level of 1 atm.
Semi-major axis(Changeable): 0.5 AU (Note that ~0.52 AU is the distance where the heat and energy received from the star is the same as Earth does from the sun), I don't know if it would be tidally-locked to the star, so just in case I made it have a very big moon with 0.2 Mass and 0.6251 Radius, so it can break the lock and make it locked to the moon instead, with a semi-major axis of 84500 km.
With ~25% land and ~75% Sea of H2CO3(Carbon Cycle caused Low CO2 on the atmosphere + Sea being H2CO3) Of course, it does have a magnetic-field and plates tectonic.
Star: (Unit is Sol/Sun = 1) Mass: 0.72 | Radius: 0.78 | Luminosity: 0.2687 | Surface Temperature: 4710 Kelvin