Question 516803
Sorry, this can't be solved to arrive at a unique solution for x and y, although z = -5.

These equations do not have one unique solution because in each of the three equations x and y are expressed in the exact same ratio (2x : -5y); therefore many (actually infinite) x and y "answers" that fit with that ratio can "successfully" be substituted into all three equations. The z terms have different ratios in relation to x and y, and therefore z can be solved via substitution and/or adding equations; resulting in z = -5; but since there is no proper solution, I didn't show work.