what is cosA=3/5

You have to use inverse cos or acos when you are looking for the angle

acos(3/5)

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{3}}}{{\mathtt{5}}}}\right)} = {\mathtt{53.130\: \!102\: \!354\: \!156^{\circ}}}$$

cosA = 3/5

A = 53.130102354156 degrees