Hi everyone, can anybody please show me how to solve these two double integration questions please?

Here they are:

(a) $\displaystyle \int_0^1 \int_0^{\arccos(x)} \cos(\sin y)} \, dy \,dx

$

(b) $\displaystyle \int_0^4 \int_{\sqrt{2}}^{3} \int_0^y z\cos (y^6) \, dx \,dy \,dz$

For the first one is painful,I cannot even start,but I guess maybe I need to change the order,am I right?

Second one is a bit horrible, I cannot continue after I integrated with respect to x, it was getting really messy.

Please show me what I should do,thanks a lot.