The area of a parallelogram is 216 cm². the height of the parallelogram is 2/3 of its base. find the base and height.

2 equations, 2 unknowns. Call height H and base B.

From the first sentence:
HB = 216

From the second sentence:
H = ⅔ B

Plug the second into the first and solve for B:
(⅔ B)B = 216
B^2 = 324
B = 18

Plug that back into either of the original 2 equations and solve for H:
H(18) = 216
H = 12