Basically what i have deduced from your info was you suspended a
300 cm string from a spring scale showing a weight of 26.69g then
you stretched the string until the scal showed 400g.
convert 400g to kg which =.4kg draw a free body diagram show the
relationship of tension to weight which mathematically looks like:
T-W=0, sum of all forces in the y direction equal to zero because
string is in static equilibrium therefor; T=w
W= .4kg*9.8m/s^2=3.92N; because a N=(Kg*m)/s^2