#2 is just the definition of a function.
#3 & # 4 are function tables. They are giving you the values for x. You substitute them into the expression and the answer is y.
SO..2x + 6 evaluated at 0 would be 2*0 + 6 = 6; 2* 1 + 6 = 8; 2 * 2 + 6 = 10
3x + 1: 3 * 0 + 1 = 1; 3 * 1 + 1 = 4; 3 * 2 + 1 = 7
#5, #6, & #7 give you the input and output (x and y) and want you to come up with the expression.
In #5 you should notice that the y's are twice the x's. So the expression is 2x. 2*3 = 6; 2*4 = 8; 2*5 = 10
In #6 The y's go up by 4 but if you multiply the x's by 4 you have one too many in the y column. This means you must subtract it. So the expression is 4x -1.
In # 7 The y's go up by 3 but if you multiply the x's by 3 you still need to add 2 more to get to the y values.
So your expression is 3x + 2.
# 8 is just y = 21x where x representing number of days.