We can solve it, but we can't actually use the interactive system they want you to use. Let's see if we can guide you through it.
My guess is you can control the lines by dragging the two points on the line.
Let's make the green line y = 2x + 3
When x=0 we get y=3, point (0,3). That's the y intercept, 3 up on the y axis. Drag one green point there.
When x=2 we get y=7, point (2,7), two to the right, seven up. Drag the other green point there. The green line is now y = 2x + 3.
Similarly the red line is made y = -3x + 3. When x=0, y=3, point (0,3). [Surprise!] When x=2, y=-3, point (2,-3). Drag the red points to (0,3) and (2,-3).
We already saw where they meet, at (0,3), x=0, y=3, the y intercept of both lines.
Answer: x=0, y=3
, where m- remaining amount of the radioactive material, x - number of hours
= 64.260 (rounded)