Question 717402
To set your proportions, always make sure both sides units match on numerator or denominator. ie cm over in = cm over in NOT cm over in= in over cm.

For you its:
distance from lightsource over illumination produced

4/75 = 9/x
Cross Multiply
4x=675
Divide both sides by 4 to get x alone
x=168.75

to check, plug the value for x back into your equation and run it through.

Don't forget to add the units for your final solution and double check the instructions on rules for rounding....168.75feet