to find the invariant line, u take 1 point from a triangle, then another point but must be the one thats stretched, eg A and A2
A= (5.6) A2=(5.12)
==(x,y)
X is invarient because it has not changed.
If it says multiply/stretch/shear triangle ABC for example by a number, and invarient line is Y, u do, for example the number is 3.
A=(5, Y=(6x3) --- == A=(5,18), and u plot that point by A3 or w/e.