Since triangles \( \triangle ABC \) and \( \triangle SRT \) are similar (indicated by \( \triangle ABC \sim \triangle SRT \)), their corresponding angles are equal.
In \( \triangle SRT \):
- \( m\angle S = 90^\circ \)
- \( m\angle R = 56^\circ \)
- \( m\angle T = 34^\circ \)
In \( \triangle ABC \):
- \( m\angle A = 90^\circ \)
- \( m\angle B \) (unknown)
- \( m\angle C = 34^\circ \)
Since the angles must sum to \( 180^\circ \) for a triangle, we can find \( m\angle B \):
For \( \triangle ABC \):
\[ m\angle A + m\angle B + m\angle C = 180^\circ \]
Substituting the known values:
\[ 90^\circ + m\angle B + 34^\circ = 180^\circ \]
Combining the constant values:
\[ 124^\circ + m\angle B = 180^\circ \]
Solving for \( m\angle B \):
\[ m\angle B = 180^\circ - 124^\circ = 56^\circ \]
Thus, the measurement of \( m\angle B \) is:
\[ \boxed{56^\circ} \]