Question 1168707
Nice job. It is quite easy to make a sign error in doing this. I made several.  Your approach is just fine.

I set (3/4)m +3/4-(m^3/4)-(1/4)=0.25 for the 25th percentile.
The first term is straightforward; the second is subtracting a -3/4, so is a +3/4
the fourth term is (-1)^3 which is -1, then subtracted (+1) because the lower integral, then - again, 3 changes of sign.

Multiplying through by 4 and you get -m^3-1+3m+3=1
m^3-3m-1=0
m(q1)=-0.347
Md m=0
m(q3)=0.347

{{{graph(300,300,-2,2,-1,1,(0.75)(1-x^2))}}}

{{{graph(300,300,-1,1,-1,1,-(x^3/4)+(3x/4))}}}