Evaluating an expression just means to replace x by its value and y by its value and do some calculus.
By hypothesis, x = 11 and y = 20.
So in the expression 14x + y, you just replace x by 11 and y by 20.
14x + y = 14 * 11 + 20 = 154 + 20 = 174
Hope this helps! :)
