Question 1033745
x is the value of a function, and t is the angle measure or variable input for the function.  You are starting with {{{x=2*cos(t)}}}.


You are interested in finding t if x=0.  You find your way to a step,
{{{cos(t)=0}}}.
At this stage, you must be acquainted with the meaning of cosine, and the common reference angles and the Unit Circle.  No more mystery.


In fact, referring to the unit circle,  {{{t=pi/2}}}  OR  {{{t=-pi/2}}}.




Same basic concepts to do your second question.