If the average of 34, 44, 28, and x is 35, what is the value of x

We have

(34 + 44 + 28 + x) / 4   = 35       multiply both sides by 4

(34 + 44 + 28 + x) = 140             simplify the left side

106 + x  = 140       subtract 106 from each side

x = 34