Question 439228
7/x+9/(x+3)=1 1/3
7/x+9/(x+3)=4/3 hrs.
Multiply all by 3(x)(x+3):
7x3x(x+3)+9x3xX=4(x)(x+3)
21x+63+27x=4x2+12x
0=4x2-36x-63
0=(2x+3)(2x-21)
x=-3/2 or 21/2
Throwing out the negative answer, we get Brett's initial speed to be 10.5 kph.  Your teacher is actually right!!!!!