12+21 divides by 3+(2 of the power of 2 times0

It is 0 hope it’s right

There exists a similar question with an attachment such that st is 9x and tu is 5x + 28. since t is the midpoint, st and tu should be equal such that,
                                      9x = 5x + 28
                                        x = 7
substituting to the known expressions,
                          st = 9x = 9(7) = 63
                            tu = 5x + 28 = 5(7) + 28 = 63
                                su = st + tu = 63 + 63 = 126
thus, the length of st and tu are both 63 and the length of su is 126.