We are given that two lines are cutting by a transversal line.
We can see that angle <1 and angle 78 degree are Consecutive Interior Angles.
Note: When two parallel lines cut by a transversal line, the each pair of angles inside the parallel lines on the same side of the transversal are called Consecutive Interior Angles.
Also please note that two Consecutive Interior Angles are supplementary.
In simple words, the sum of two Consecutive Interior Angles is 180 degree.
So, we can apply supplementary angles property of Consecutive Interior Angles to find the angle <1 and given angle 78 degrees in the given diagram.
Therefore, <1 + 78 = 180°.
Subtracting 78 from both sides, we get
<1=102°.
Therefore, <1=102°.