There are 13 stripes so at 3.5 inches for each stripe the flag will be 13(3.5)= 45.5 inches wide, To be 3x5, the length will have to be (5/3)(45.5)= 75.8... which I would round to 76 inches long.
The width (height) of the union is 7/13 the width of the flag so here it would be (7/13)(45.5)= 24.5 inches wide.
The length of the union is 0.35 the length of the flag so here it would be (0.35)(76)= 26.3 inches long.
There are, of course, 50 stars in the union, arranged in five rows of six stars and, between them, four rows of five stars. Each star would be 2.45 inches in diameter.
Three red stripes and three white stripes run the entire length of the flag so are 3.5 inches by 76 inches.
The other four red stripes and three white stripes run from the union to the end of the flag so are 3.5 inches by 76- 26.3= 50 inches long.