The sum of first three terms of an arithmetic series is 21.the sum of first two terms is subtracted from the third term ,then it would be 9,find first the three terms of the series

Can you answer my question? will mark brainiest!

Which transformations could have occurred to map △abc to △a"b"c"? a rotation and a reflection a translation and a dilation a reflection and a dilation a dilation and a rotation

Long division setup showing an incomplete calculation. 12 is in the divisor, 6839 is in the dividend, and 5 hundreds and 6 tens is written in the quotient. 6000 is subtracted from 6839 to give 839. an unknown value represented by a box is being subtracted from 839. what number should be placed in the box to complete the division calculation?