Answer:
<h2>A) 3 cm</h2>
Step-by-step explanation:
The formula of a volume of a cylinder:

r - radius
H - height
We have r = 1cm and H = 21cm. Substitute:

The formula of a cone:

r - radius
H - height
We have V = 21π cm³ and H = 7cm. Substitute:
<em>divide both sides by π</em>
<em>divide both sides by 7</em>
<em>multiply both sides by 3</em>

Df = g - 10
To solve for d, divide both sides by f.
df / f = (g - 10) / f
d = (g - 10) / f
I hope this explains it.
A is your answer the this question