5
令cosx=3/5,sinx=4/5
y = 3sinα+4cosα
=5*(3/5sinα+4/5*cosα)
=5*(sinαcosx+cosαsinx)
=5*sin(α+x)
-1<=sin(α+x)<=1
-5<=5sin(α+x)<=5
所以y的最大值是：5
x=arcsin3/5