Variation

Let y = k/x, where k is not equal to 0.

(a)
Put x = a and y = 3.
3 = k/a ...... (1)

Put x = b, y = 4.
4 = k/b ...... (2)

(2)/(1), we have
4/3 = a/b
So a : b = 4 : 3.

(b) From (2), we have k = 4b.
Put x = a + b and k = 4b.
y = 4b/(a + b)
1/y = (a + b)/4b
4/y = (a + b)/b
4/y = a/b + b/b
4/y = 4/3 + 1 [from (a))
4/y = 7/3
y = 12/7