Greetings from Brasil...
We will answer in a different way. Using knowledge about proportion.
If we have a proportion, then we have the same numerical value that multiplies both the numerator and the denominator. Let X be this number.
b/h = 3/4
where
b = base
h = altitude
So,
b = 3X
h = 4X
For the area of the parallelogram:
A = b·h
as b = 3X and h = 4X, then
A = 3X · 4X
A = 12X²
X = √(A/12) as A = 1200
X = √(1200cm²/12)
X = 10cm
b = 3X ⇒ b = 3·10cm
b = 30cm
h = 4X ⇒ h = 4·10cm
h = 40cm
