Question

I really need a quick answer

A sector of a circle of radius 7cm subtending an angle 270 degree at the centre of the circle is used to form a cone

Answer 1

for the cone, the base circumference is 3/4 the circumference of the circle.

so, its base radius is also 3/4 the radius of the circle.
The slant height of the cone is the radius of the circle.

See what you can do with that, since you didn't actually ask a question.