Let y represent the distance from your house to wawa. This means that the distance from wawa to your house is also y miles.
It takes you 0.4 hours at x miles per hour to make it to Wawa.
Distance = speed * time
Therefore, on your way to wawa, the expression would be
y = 0.4 * x = 0.4x
On your way back you have to ride 3 miles per hour slower. This means that your speed was (x - 3) miles per hour. Since it took you 0.5 hours, the expression for distance would be
y = 0.5(x - 3)
Since the distance is the same, it means that
0.4x = 0.5(x - 3)
0.4x = 0.5x - 1.5
0.5x - 0.4x = 1.5
0.1x = 1.5
x = 1.5/0.1
x = 15
Therefore,
y = 0.4x = 0.4 * 15
y = 6
The distance from your house to wawa is 6 miles