Due to the blue lines are parallel, you can write the following proportions:
(x + 5)/21 = (x + 3)/15
solve for x as follow:
multiply both sides by the LCM OF 21 and 15, which is 21x15 = 315:
(x + 5)(315/21) = (x+3)(315/15)
15(x + 5) = 21(x+3) apply distribution property
15x + 75 = 21x + 63 subtract 15x and 63 both sides
75 - 63 = 21x - 15x simplify like terms both sides
12 = 6x divide by 6 both sides
12/6 = x
2 = x
x = 2
Hence, the value of x is x = 2