[{(39.2)^(2)*sin(x)^(2)}+980]/(2*9.8)={(39.2)^2*cos(x)^2}/(*·9.8)

Should the denominator of the right hand side be "2 * 9.8"? If so, the denominators of both sides cancel, and you can rearrange this to be of the form C = cos^2x - sin^2c, where 'C' is a constant. Recall that cos(2x) = cos^2x - sin^2x, so this simplifies to an equation of the form cos(2x) = C, and you can solve for x by taking the arccosine of both sides.