Replacing nulls with a date from another column, but only if a third column is not blank

[SJQuick]

I am trying to replace blank dates with a date from another column, but only if another column is populated with a date. Per the example below, if the Initiate Date is blank replace with Alternate Initiate Date if Complete Date is not blank, e.g. ID 300 and 800. Please not the Alternate Initiate Date was created using a LOOKUP.

I would greatly appreciated any help, as I've been working on this for a while trying many different ways. Everything that I've found on the Internet seems to leave out the "if only another column is not blank" part and/or the LOOKUP column is not selectable in the formula.

[Ahmedx]

watch my video you will learn how easy it is to do it in power query
https://1drv.ms/v/s!AiUZ0Ws7G26RhmzwljmUwaso5szz?e=Q6hL8L

[SJQuick]

Thank you for the video @Ahmedx. I could not get your solution to work in power query due to the alternate date being the product of a LOOKUP, but I have taken note and saved for future use. Have a great day!

[vicky_ · Accepted solution]

Create a new column (either in PQ or DAX). 
For DAX: IF(NOT(ISBLANK([Complete Date])) && NOT(ISBLANK([Initiate Date])), [Alternative Initiate Date], [Initiate Date])

[SJQuick]

Thank you so much @vicky_ ! This worked with one minor tweak (removed the second NOT). Have a fabulous day!