Andrea alternates between major and minor thirds on the same string, then plays a dominant seventh starting on 1st finger E on the D string, in the key of A major.
A common intonation problem in dominant sevenths is that the two different positions of the changing finger are not clearly differentiated. When the starting note is played with the 1st finger, as here, the changing finger is the 3rd finger.
By transferring the two positions of the 3rd finger (High, Middle) on to one string you can hear the difference and practise the change.
Finger Patterns Demonstration
Scores - Major & Minor 3rds for Dominant 7th - violin & viola