Zeta Corporation's taxable income for 2017 was $1.1 million, on which Zeta paid federal income taxes of $374,000 (at its then 34% tax rate). Zeta estimates calendar year 2018's taxable income to be $2 million, on which it will owe $420,000 in federal income taxes.

a. What are Zeta's minimum quarterly estimated tax payments for 2018 to avoid an underpayment penalty?

b. When is Zeta's 2018 tax return due?

c. When are any remaining taxes due? What amount of taxes are due when Zeta files its return assuming Zeta timely pays estimated tax payments equal to the amount determined in Part a?

d. If Zeta obtains an extension to file, when is its tax return due? Will the extension permit Zeta to delay making its final tax payments?