Step 1: Understanding motion of particle A.
Particle A moves along positive x-axis with speed \(v\), so:
\[
x_A = vt, \quad y_A = 0
\]
Step 2: Understanding motion of particle B.
Particle B moves along positive y-axis with speed \(v\), so:
\[
x_B = 0, \quad y_B = vt
\]
Step 3: Coordinates of both particles at time t.
So at time \(t\):
\[
A(vt, 0), \quad B(0, vt)
\]
Step 4: Equation of line joining two points.
Slope of AB:
\[
m = \frac{vt - 0}{0 - vt} = -1
\]
Using point A:
\[
y - 0 = -1(x - vt)
\]
Step 5: Simplification.
\[
y = -x + vt
\]
Step 6: Final conclusion.
Hence, equation of line is:
\[
\boxed{y = -x + vt}
\]