In order to determine the missing coordinate in the ordered pair (?,4), replace y=4 into the given equation and solve for x, as follow:
5x + 4y = 12
5x + 4(4) = 12
5x + 16 = 12 subtract 16 both sides
5x = 12 - 16 simplify
5x = -4 divide by 5 both sides
x = -4/5
Hence, the missing coordinate is -4/5