(X₁-X) (Y₁-Y)+(x₂-x)(V₂-Y)=(√((X₁-X)²+(x₂-x)²))(√((Y₁-Y)²+(y2-V)²))

Solve equation for y2 in terms of the other variables with one exception. Here is the substitution for y:

y = (y1+Y₂)/2