4) given is that
m<DAC=y
m<BAC=3x
ABCD is a square so m<DAB = 90° because each angle in a square is a right angle so 3x+y=90° (1) ;
AC is a diagonal in the square so m<DAC=m<BAC so 3x=y (2);
Find x by substituting y to 3x from (2) into (1)
3x+3x=90
6x=90
x=90/6=15°
Answer x=15°