you know that if x=a, then when @=0, y=0, and when @ = pi/2, sin[y] = 1/rt5 and cos[y] = 2/rt5 (just from looking at the right angle triangle NTO)
so subbing into the equation for dy/dt;
when @ = 0
dy/dt = ucos(0-0)/(2acos(0) + acos(0-0))
= u/3
when @ = pi/2
dy/dt = ucos(pi/2 - y)/(2acos[y] +...