You can try something like this:

# The predictor vector.
a <- c(111, 123, 134, 156, 145, 176, 198, 133, 131, 181)
# The resposne vector.
b <- c(56, 61, 65, 71, 45, 54, 74, 76, 68, 58)
# Apply the lm() function.
relation <- lm(a~b)
# Find weight of a person with height 170.
z <- data.frame(x = 170)
result <- predict(relation,z)
print(result)