Let the age of Gabrielle be G
and the age of Mikhail age be M
given: Gabrielle is 15 years younger than Mikhail → G = M - 15
the sum of their ages is 43 → G + M = 43
M - 15 + M = 43
2M = 43 + 15
M = 58/2
M = 29
G = 29 – 15 = 14
Mikhail is 29 years old.