Question 902863
You did not show/give exactly what you meant.
{{{graph(300,300,-4,4,-4,4,3/4x-1,-4/3x-1)}}}



Use proper grouping symbols to express the numbers exactly how you mean:
{{{system(y=(3/4)x-1,y=-(4/3)x-1)}}}


You can clearly read the slopes and y intercepts, and obviously slopes being negative reciprocals,

{{{graph(300,300,-4,4,-4,4,(3/4)x-1,-(4/3)x-1)}}}