Image of A(a,b) along y=x is P′(b,a).
Translating it 2 units along positive direction of x−axis it becomes P"(b+2,a).

Now, applying Rotation theorem
−21+27i=((b+2)+ai)(cos4π+isin4π)
Multiplying the complex numbers we get,
2−1+27i=(2b+2−2a)+i(2b+2+2a)
⇒b−a+2=−1...(1)
and b+2+a=7...(2)
⇒a=4;b=1
⇒2a+b=9