Okay multiply the coefficients of x² and the constant;
 
3×4=12 
How can we split -13 so that the numbers multiply to give 12?
-12×(-1)=12 and -12-1=-13 
Therefore, rewrite the equation; 
3y²-1y-12y+4
y(3y-1)-4(3y-1) <-- make sure everything inside the brackets remains the same
(y-4)(3y-1) 
Double check by expanding; 
(y-4)(3y-1)
3y²-1y-12²+4 
3y²-13y+4 <-- this is the same equation, meaning we were correct. 
Hope I helped :)