Use addition or substitution to find the value of x for this set of equations 4x plus 13y equals 40 4x plus 3y equals -40?

Try elimination.

4X + 13Y = 40

- 1(4X + 3Y = - 40)

4X + 13Y = 40

- 4X - 3Y = 40

-----------------------add

10Y = 80

Y = 8

-------------find X ( top equation )

4X + 13(8) = 40

4X + 104 = 40

4X = - 64

X = - 16

----------------------check in both equations for consistency

4(- 16) + 13(8) = 40

- 64 + 104 = 40

40 = 40

-------------top checks

4(- 16) + 3(8) = - 40

- 64 + 24 = - 40

- 40 = - 40

-------------------both check and are consistent

So, X is,

- 16 ( while Y is 8 )