Answer:
a)  C = A cos φ , S = A sin φ Â
Explanation:
a) The two equations given are equivalent ,
we know the formula of Â
      Cos (a + b) = cos a cos b - sin a sin b
let say
     a  = ωt   and    b = φ
     x(t) = A (cos ωt cosφ- sin ωt sin φ )
the second equation is
     x(t) = C cos ωt + S  sin ωt
they both are equivalent
     C cos ωt + S sin ωt = A cos φ cosωt - a sin φ sin ωt
The coefficients of the equation should be equal
    C = A cos φ
     S = A sin φ