what is the value of x when the adjacent is 7cm and the opposite is 15cm?

I assume that by x you mean the angle of x. You can use the trigonometric function tan to find the angle.

atan(15/7) which is equal to approximately 65 degrees. ( Rounded to nearest whole number)

Thank you Quazars,  

X is an angle so you have to use inverse trig.

x=tan^-1(opp/adj)=tan^-1(15/7)

Inverse tan is the same as arc tan so on the web2 calculator it is

atan(15/7)

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{15}}}{{\mathtt{7}}}}\right)} = {\mathtt{64.983\: \!106\: \!521\: \!9^{\circ}}}$$