Answer :
4r +1y =7...eqn 1
3r+1y = 6...eqn 2
Make y subject of formula for eqn 2...
y = 6-3r...eqn 3
Subst eqn 3 into eqn 1...
4r +6-3r=7
r =1
Subst r=1 into eqn 3
y = 6-3=3
Red roses cost $1. each
Yellow roses cost $3 each
3r+1y = 6...eqn 2
Make y subject of formula for eqn 2...
y = 6-3r...eqn 3
Subst eqn 3 into eqn 1...
4r +6-3r=7
r =1
Subst r=1 into eqn 3
y = 6-3=3
Red roses cost $1. each
Yellow roses cost $3 each