15/ X = 3/(x-7)

Multiply both sides by x(x-7) you will get

15(x)(x-7)/x = 3(x)(x-7)/(x-7)

15(x-7) = 3x

15x-105=3x

Subtracting 3x from both sides you will get

15x -3x -105 = 3x - 3x

12x -105 = 0

Adding 105 on both sides you will get

12x - 105 +105 = 105

12x = 105

Dividing both sides by 12 you will get

12/12x = 105/12

x = 35/4

Multiply both sides by x(x-7) you will get

15(x)(x-7)/x = 3(x)(x-7)/(x-7)

15(x-7) = 3x

15x-105=3x

Subtracting 3x from both sides you will get

15x -3x -105 = 3x - 3x

12x -105 = 0

Adding 105 on both sides you will get

12x - 105 +105 = 105

12x = 105

Dividing both sides by 12 you will get

12/12x = 105/12

x = 35/4