The question is not completely unambiguous as it’s not mentioned that lengths of strings are natural numbers. Lengths of strings are usually continuous rather than discrete in nature so it should have been mentioned in the question that lengths are natural numbers. Anyways, going by this assumption, and supposing that A,B,C,D,E,F, and G are those lengths in ascending order,
(A+B+C+D+E+F+G) = 68 * 7 = 476 -- (Eqn 1)
Median D = 84. Now, for G to be as high as possible, E and F should be as close to median D as possible. That requires E = F = 84.
Replacing these values and also G = 4A + 14, in Eqn 1
ð A+B+C+84+84+84+4A+14 = 476
ð 5A + B + C = 210 --- (Eqn 2)
Now, since G is directly proportional to A, the higher the A the higher the G. For A to be as high as possible, B and C in Eqn 2 should be as low as possible. Lowest possible value of B as well as C is A. So
5A + A + A = 210
ð A = 30
ð G = 4 * 30 + 14 = 134