The first thing we need to note is that we know the value of all triangle's edges. So we have many options to find ?.
First option) The first option is the tangent.
[tex]\tan (?)=\frac{opposite\text{ side}}{adjacent\text{ side}}=\frac{21}{20}[/tex]
It means that
[tex]\tan ^{-1}(\frac{21}{20})=?[/tex]
Putting the left-hand side in a calculator we get
[tex]\text{?}\approx46.39\degree[/tex]
Because the first decimal place (3) is less than 5, the nearest whole degree is 46°.
