The rule Ro,270 T(4,-1) means that first point M is translated 4 units to the right and 1 unit down, and then is rotated about the origin 270°.
T(4, -1) transforms point (x, y) into (x+4, y-1):
M(-3, 4) → (-3+4, 4-1) → M'(1, 3)
Ro,270 transforms point (x, y) into (-y, x):
M'(1, 3) → M''(-3, 1)
Then, the y-coordinate of M'' is 1