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M = J - 4 (1) ("Marty is four years younger than Jennifer now.")
M+10 = 2J (2) ("in 10 years Marty will be twice the age that Jennifers now.)
Substitute expression (1) into eq(2). You will get
J-4 + 10 = 2J ====> J = 6.
Then M = J-4 = 2.
Answer. M = 2, J = 6.