a^3 + b^3 = 42
Note that this factors as
(a + b) (a^2 - ab + b^2) = 42
( 7) (a^2 - ab + b^2) = 42 divide both sides by 7
a^2 - ab + b^2 = 6 (1)
a + b = 7
square both sides
a^2 + 2ab + b^2 = 49 (2)
Subtract (1) from (2)
3ab = 43
ab = 43/3
And
1/a + 1/b =
( b + a) / ab = ( a + b) / ab = 7 / (43 / 3) = 7 * 3 / 43 = 21 / 43