|nbrane: i've never worked with C++ before|
I'm not about to write someone's homework for them (unless they spend the time convincingly obscuring their question), so I'll just point out what's going wrong.
var count = 1
Arrays are 0 indexed, not 1.
count <= productArray
productArray is never used. I assume you're supposed to store the length of the product array before looping. Also, the arrays are 0 indexed, not 1 indexed.
product = plantHeights * plantNumbers;
I'm assuming you're trying to take the plantHeights element, add the corresponding plantNumbers element to it, and store the result in the relevant product element, but that's not what you're doing. I suggest you reread the chapter in your textbook, or read this page at OpenJS.