$(15$ points$)$ Textbook, Ch $1,$ Problem $43$

Find the angle between each of the following pairs of vectors.

a$.$ vec$(A)=-2\mathrm{.}00$hat$()+6\mathrm{.}00$hat$()$ and vec$(B)=2\mathrm{.}00$hat$()-3\mathrm{.}00$hat$().$

b$.$ vec$(A)=3\mathrm{.}00$hat$()+5\mathrm{.}00$hat$()$ and vec$(B)=10\mathrm{.}00$hat$()+6\mathrm{.}00$hat$()$

c$.$ vec$(A)=-4\mathrm{.}00$hat$()+2\mathrm{.}00$hat$()$ and vec$(B)=7\mathrm{.}00$hat$()+14\mathrm{.}00$hat$().$