Explanation:
I think the actual equation you meant to write was 5x⁴ + 7x³ - 2x² - 3x + c.
You're going to equate the whole equation to 0 and replace x for -1( because x + 1 is a factor)
= 5(-1)⁴ + 7(-1)³ - 2(-1)² - 3(-1) + c = 0
= 5 - 7 - 2 + 3 + c = 0
= - 1 + c = 0
Therefore c = 1