sinx = -2/(5^0.5)

3pi/2 < x < 2pi

the question that i donīt understand is that i shuold use this values and decide the exacly value of this expression: cos(2x-3pi/4)

so i did this way

arcsin(-2/(5^0.5) = x = -63.4

cos(2*(-63.4)-3pi/4)

are i doing right ?