PLS HURY 35 POINTS

What is the correct cross product that he should use to solve for the missing dimension?
(8)(12) = 21x
(8)(21) = 12x
(12)(8) = 12x
(12)(21) = 8x

PLS HURY 35 POINTS What is the correct cross product that he should use to solve for the missing dimension 812 21x 821 12x 128 12x 1221 8x class=