If m 3= 24 find each measure

Can someone check if i did this correct. it’s number 4 if you’re wondering.

Asystem of equations has 1 solution. if 4x-y=5 is one of the equations , which could be the other equation ?

Scores on the cognitive scale of the bayley-scales of infant and toddler development range from 1 to 19, with a standard deviation of 3. calculate the margin of error in estimating the true mean cognitive scale score, with 99% confidence, if there are 12 infants and toddlers in a sample.