How do I find the limit to an intersection of two functions?

I have this exercise to find the limit to an intersection of two functions I have no idea how to do it, some gidance would be helpful

Two material points move towards each other through two laws of motion given by y1(t) = a − sin(t), y2(t) = t, 'a' being a real parameter. We designate by τa ∈ R the instant when the two trajectories intersect. Determine an interval Ia = [b1, b2], 0 < b1 < b2, of admissible values for the parameter a, in order to guarantee that τa ∈ I = [1, 2].

I'm told I have to use Bolzano's Theorem, but still I don't know how to apply it.

[Torsten]

Hint: Consider

1 + sin(1)

ans = 1.8415

2 + sin(2)

ans = 2.9093

[Bjorn Gustavsson]

Just plot the two functions for a couple of parameter-combinations. This will give you some visual/physical insight in what the functions actually mean. Then have a think.