Question 122739
252/(x+7)+252/x=21
[252x+252(x+7)]/x(x+7)=21
[252x+252x+1764]/(x^2+7x)=21 now cross multiply.
21x^2+147x=504x+1764
21x^2+147x-504x-1764=0
21x^2-357x-1764=0
21(x^2-17x-84)=0
21(x-21)(x+4)=0
x-21=0
x=21 return trip speed.
21+7=28 for the going speed.
proof
252/28+252/21=21
9+12=21
21=21