(a) He should place the sheet of paper at the focus of the mirror to burn the paper.
(b) He has a concave mirror.
(c) Yes, the sheet of paper will start burning at the focus of the mirror which will give approximate value of focal length, i.e., the distance between mirror and the focal point where the sheet of paper starts burning.A concave mirror forms a real image of the sun.
ORGiven : Height of object, h1=+10‌cm. Focal length, f=+12‌cm.
Object distance, u=−18‌cm. From the lens fomula,
‌

1

v

−‌

1

u

‌=‌

1

f

‌

1

v

−‌

1

−18

‌=‌

1

12

‌

1

v

+‌

1

18

‌=‌

1

12

‌

1

v

‌=‌

1

12

−‌

1

18

‌

1

v

‌=‌

3−2

36

=‌

1

36

v‌=36‌cm
‌ fication, ‌m‌=‌

h2

h1

=‌

v

u

=−2
h2‌=‌

v

u

×h1
h2‌=‌

36

−18

×10‌cm
‌=−20‌cm
The position of image formed is at distance of 36‌cm from convex lens.
Since the value of magnification is more than 1 (it is 2), the image formed is larger than object.
The minus sign of magnification shows that image is formed below the principal axis. Hence, the image formed is real and inverted.