P=Perimeter
W=Width
L=Length
Assuming the 210 feet is the perimeter
P = 210 feet
P = 2W + 2L
W = 2/3 L . . . so sub W with 2/3 L in the equation
P = 2(2/3 L)+ 2(L) = 210 feet
4/3 L + 2L = 210 feet
10/3 L = 210 feet
Multiply both sides by 3/10 to solve for L
L = (210)(3/10) = 63 feet
W = 2/3 L so W = 2/3 x 63 = 42 feet
Garden is 42 feet wide by 63 feet long