What is the correct also how do i solve it? ?

Since it is a right angle triangle
use Pythagoras theorem
20^2 = 3x^2 + 4x^2
400 = 9x^2 + 16x^2
400 = 25x^2
x^2 = 400/25
x^2 = 16
x = 4

Das is right triangle, this means taht
if hyptonuse is c and legs are a and b then
a²+b²=c²
so

c=hypotonuse=20
the legs are 3x and 4x
so
(4x)²+(3x)²=20²
16x²+9x²=400
25x²=400
divide both sides by 25
x²=16
sqrt both sides
x=16