Let x and y be the two parts.
We know that x + y = 21.
x = (3/4)y one is 3/4 of the other
Now use the first equation and substitute the second equation into it.
x + y = 21
(3/4)y + y = 21
Now combine like terms...
(7/4)y = 21
y = 21/(7/4) can be re-written as 21/1.75 = 12
So if y = 12, look at either equation and substitute in 12 in place of y.
What is x?