Polar & rectangular forms of complex numbers

Problem

Express z, start subscript, 1, end subscript, equals, 10, open bracket, cosine, left parenthesis, 270, degree, right parenthesis, plus, i, sine, left parenthesis, 270, degree, right parenthesis, close bracket in rectangular form.
Express your answer in exact terms.
z, start subscript, 1, end subscript, equals