As we know that range of the projectile motion is given by
[tex]R = \frac{v^2 sin(2\theta)}{g}[/tex]
here we know that range will be same for two different angles
so here we can say the two angle must be complementary angles
so the two angles must be
[tex]\theta, 90 - \theta[/tex]
so it is given that one of the projection angle is 75 degree
so other angle for same range must be 90 - 75 = 15 degree
so other projection angle must be 15 degree