Answer:
-9.3x + 13.1y
Explanation:
(3.5x - 2.7y) + (-12.8x + 15.8y) = 3.5x - 2.7y - 12.8x + 15.8y =
= 3.5x - 12.8x - 2.7y + 15.8y = (3.5 - 12.8)x - (2.7 - 15.8)y =
= -(12.8 - 3.5)x + (15.8 - 2.7)y = -9.3x + 13.1y
because
-(-a) = a
-(a + b) = -a - b
-(a - b) = -a + b = b - a
a + (-b) = a - b