a = 15
a4 = 61
difference between 2 consecutive terms (d)
= a2 - a = 23 - 15 = 8
Now,
Next three terms shall be a5, a6, a7.
a5 = a + 4d = 15 + 4×8 = 15 + 32 = 47
a6 = a + 5d = 15 + 5×8 = 15 + 40 = 55
a7 = a + 6d = 15 + 6×8 = 15 + 48 = 63
Hence, next three terms are 47, 55, 63.