You need to convert this problem to a standard normal distribution to calculate the probability. That means you convert the systolic number 120 to a Z-score. Z = ( X - μ ) / σ, and you know all three variables.
Z = (120 - 120)/8 = 0/8 = 0.
P(X > 120) = P(Z > 0) = 0.5000, which can easily be looked up on a standard normal distribution probability table, or hopefully you remember that the normal curve is symmetric around Z=0 such that half of the area under the curve falls on either side of Z=0.