The solution is x=arcsin(0.25), so we need to find an equation with the solution 3arcsin(0.25).
So x/3=arcsin(0.25) and sin(x/3)=0.25.
3sin(x/3)-4sin³(x/3)=0.25. This cubic equation needs to be solved for x/3.
It can be written 16sin³(x/3)-12sin(x/3)+1=0.
If y=x/3, 16sin³y-12siny+1=0 is an equation with the required solution given by y.
x=arcsin(0.25)=0.25268 or 2.88891 approx, so y=x/3=0.084227 or 0.96297, siny=0.0841272 or 0.820892 which is a solution of the cubic. The third solution to the cubic is siny=-0.905019, y=5.151761, if the domain applies to y.