# Determining distance from object in concave lens [closed]

so I was studying for my Yr11 Physics test tomorrow and I came across a question that I got the wrong answer on, all of my friends are getting the same answer.

Question: You wish to project the image of a lamp, magnified three times, onto a screen 5.0m from the lamp. How far from the lamp should the mirror be placed?

I though that we would just use $M=\frac{-d_i}{d_o}$ where $M=3$ and $d_i=5$, which will work out that $d_o\approx -1.7$ which has a magnitude of 1.7m from the lamp

What have I done wrong? Also, please keep in mind that I am in year 11 and may not understand anything beyond that level

## closed as off-topic by John Rennie, jinawee, tpg2114♦, Qmechanic♦Dec 1 '13 at 13:49

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• How far from the lamp should it be placed? What is the "It" – DJohnM Dec 1 '13 at 7:55
• There was a question before it saying What kind of spherical mirror is needed? which I know is concave, so the "it" is a concave mirror – VikeStep Dec 1 '13 at 7:56

The lamp is $x$ m from the mirror; the image is $5$ m from the lamp, which puts it $(5+x)$ from the mirror. These are the d values for your magnification formula, which is correct...