Convert the angle to radians: 31/60×π/180 then multiply by 360000=3246km approx.
If you’re not very mathematically minded think of it like this:
31 minutes is about half a degree, and there are 360 degrees in a circle, so half a degree is about 1/720 of the circumference of a circle. The moon’s orbit is approximately circular with a radius of 360000km, so the circumference of the orbit of the moon is about 720000π km (2πr). The diameter of the moon (width) is therefore 720000π/720=1000π km. And the value of π is roughly 3.14. So that is 3140 km. 31 minutes is slightly more than half a degree so the moon’s diameter is slightly more (31/30) than 3140km, and that is the answer we got: 3246km.
Note that when an angle is small, the sine and tangent of the angle and the angle itself (in radians) are approximately equal.