I hope this helps you
cos75=cos (45+30)
cos45.cos30-sin45.sin30
square root of 2/2.square root of 3/2-square root of 2/2.1/2
square root of 6/4-square root of 2/4
0,61-0,35
0,26
y^2=x^2+z^2-2.x.z.cos75
y^2=25^2+ (10,5)^2-2.25.10,5.0,26
y^2=625+110,25-136,5
y^2=598,75
y~24,4