What exactly are you looking for?
This would just be 7 1/5 + 3/5 since the negative sign gets distributed and makes (-3/5) into a positive number.
From there you have basic math and you get 7 4/5.
Let x be the lengths of the steel rods and X ~ N (108.7, 0.6)
To get the probability of less than 109.1 cm, the solution is computed by:
z (109.1) = (X-mean)/standard dev
= 109.1 – 108/ 0.6
= 1.1/0.6
=1.83333, look this up in the z table.
P(x < 109.1) = P(z < 1.8333) = 0.97 or 97%
