How is the temperature of stars determined?

$A$ black body is at a temperature of $5780 \text{ K}$. The energy of radiation emitted by the body at wavelength $300 \text{ nm}$ is $U_1$, at wavelength $500 \text{ nm}$ is $U_2$, and at wavelength $900 \text{ nm}$ is $U_3$. Wien's constant $b = 2.89 \times 10^6 \text{ nm K}$. This shows that:

$A$ black body is at a temperature of $2880\;K$. The energy of radiation emitted by this object with wavelength between $499\;nm$ and $500\;nm$ is ${U_1}$,between $999\;nm$ and $1000\;nm$ is ${U_2}$ and between $1499\;nm$ and $1500\;nm$ is ${U_3}$. The Wien's constant $b = 2.88 \times {10^6}\;nm\,K$. Then

Assume that for solar radiation,the surface temperature of the Sun is $6000 \,K$. If Wien's constant '$b$' is $2.897 \times 10^{-3} \,m-K$,the value of the maximum wavelength will be: (in $Å$)

$A$ black body is at a temperature of $5760 \ K$. The energy of radiation emitted by the body at wavelength $250 \ nm$ is $U_1$,at wavelength $500 \ nm$ is $U_2$ and that at $1000 \ nm$ is $U_3$. Wien's constant,$b = 2.88 \times 10^6 \ nm \ K$. Which of the following is correct?

The absolute temperatures of two black bodies are $2000 \ K$ and $3000 \ K$ respectively. The ratio of wavelengths corresponding to maximum emission of radiation by them will be