test statistic z = (sample mean - population mean)/(population standard deviation/square root(sample size))
You are given sample mean = 91.8, population mean = 90, population standard deviation = 10.26 and sample size = 100.
p-value is P(z > computed value) + P(z < - computed value) = 2*P(z < -computed value), which you can look up from standard normal probability table or using a statistical calculator or Excel.