15x+25=5(3x+5)
We need to find a common denominator, which is 5. Then 5 times how much gives us 15 and 25. In this case, 5 times 3 gives us 15 and 5 times 5 it's 25. The x is there because 5 times 3x gives us 15x, or another way is: 15x divided by 5 is 3x .
Hope I helped