105:(p+3)=21 tolong yah aku gak ngerti???

105:(p+3)=21
105 = 21(p+3)
105 = 21p + 63
21p = 105 - 63
21 p = 42
p = 42 / 21
p = 2

105 × 3/p = 21
315/p       = 21
p              = 315 / 21
p              = 15