Somebody help me ??!!

*this answer i made was incorrect please avoid it*

15

Step-by-step explanation:

Pythagoras theorem

a^2+b^2=c^2

12*12=144

9*9=81

144+81=225

sq root of 225 = 15

A=20,000 m2 per year

Step-by-step explanation:

we have that Volume (V) =300,000 m3, so V=A*h, where A=area (m2) and h=15m

V=A*h ⇒ 300,000m3=A*15m ⇒ A=300,000m3/15m ⇒ A=20,000 m2