Assuming that we are in degrees

oos(21)   = [5x + 27]/ [15x]   multiply both sides by 15x

(15x) cos(21)   5x + 27     subtract 5x from both sides

(15x) * cos(21) - 5x   = 27    factor x out of the left side

x * (15cos (21) - 5)  = 27      divide by   (15cos (21) - 5)

x  =   (27)  / (15cos (21) - 5)   ≈ 2.998 ≈  3