You are not correct, the right answer should be 5-2 square root symbol. I couldn't find out how to put one there.
First you need to divide 125 / 25 which is 5 then you do subtraction 21 x 5 which is 105, then you subtract again 125 - 105 which is 20, 20 light bulbs will be broken. Or you could have just multiplied 5 by 4 which is the amount of light bulbs that were broken which is a lot easier...
Z - 7 = 8
subtract 7 from both sides
z = 15
hope this helped!