Answer:
a. y = 17.5x
b.

c. 4x + 8
Explanation:
a.
Beef costs $17.50 per kg. That is a unit rate. To find the price of an amount x of beef, multiply the amount, x, by the unit rate.
y = 17.5x
b.
I think you may mean "add," not "and."
"Write an expression for the number that results when you think of a number 3x, add 5 to it and take the square root of the result."
The number is 3x.
Add 5 to it to get 3x + 5.
Take the square root to get

c.
The width is x. The length is 4 cm more than x, so it is x + 4.
Length: x + 4
Width: x
perimeter = length + length + width + width
P = 2L + 2W
P = 2(L + W)
P = 2(x + 4 + x)
P = 2(2x + 4)
P = 4x + 8