please help to solve these problems

1. when an object is placed 12 cm from a converging lens, an image if formed 36 from lens. what is focal length of this lens?

a. -4cm

b. 4cm

c. -0.11 cm

d. 0.11 cm

e. this lens has no focal length since it is a diverging lens.

2. In a graph of 1/d0 versus 1/di both the x and the y intercepts represent

a- the focal length

b-infinity

c-1/f

d- the optic axis

e-the magnification of the lens

3. The focal length of a certain lens is known to be 19.4cm. A prospective Nursing student in lab measures the focal length to be 23.7 cm. What is the percent error of this measurement?

a. 22.2%

b. 11.7%

c. 18.1%

d. -22.2%

e. -18.1%