I believe it is called the "Lunar Crust" but not 100% sure.
Answer:
15.75 cm
Explanation:
focal length of convex lens, f = + 9 cm
position of object, u = - 21 cm
Let the distance of image is v.
Use lens equation
1 / f = 1 / v - 1 / u
1 / 9 = 1 / v + 1 / 21
1 / v = 1 / 9 - 1 / 21 = (21 - 9) / (21 x 9)
1 / v = 12 / 189
v = 15.75 cm
STEP ONE:
Let you and your friend stand as far away as possible from a large reflecting wall and clap your hands rapidly at a regular rate.
STEP TWO:
Adjust this rate until each clap just coincides with the return of an echo of its predecessor, or until clap and echo are heard as equally spaced.
STEP THREE:
Use a stopwatch to find the time between claps, t. Make a rough measurement of distance to the wall, s. Thus the speed of sound, v = 2s/t
it will be very hot so that causes nobody to walk on venus because its the hottest outer planet.
hope I helped :)