Calculate the PMOS device with to acheive symmetric propagation delays in an inverter (tPLH = tPHL). Use Wn=450nm Ln =300nm and Lp=300nm and solve for Wp. So I think by some ratio I can find what Wp should be, maybe something like Wn/Ln=Wn/Lp which would mean Wp=450nm I am not 100% sure.