I need help with these three questions and also if there is a formula can you tell me what it is. Thank You I need help because I don't understand this.

Which linear equations have an infinite number of solutions? check all that apply.  (x  –  3/7)  =  2/7(3/2x  –  9/14)8(x  +  2)  =  5x  –  1412.3x  –  18  =  3(–6  +  4.1x)(6x  +  10)  =  7(x  –  2)4.2x  –  3.5  =  2.1 (5x  +  8)

If x is 6, then 7x= a. 13 b. 42 c. 42x d. 76

How can constraints be used to model a real-world situation?