Explanation

Given:

(x + 5) : (x + 7) = 5 : 7

We can write it as:

(x + 5) / (x + 7) = 5/7

Cross-multiplying:

7(x + 5) = 5(x + 7)

7x + 35 = 5x + 35

7x - 5x = 35 - 35

2x = 0

x = 0