You need to use a trig function to solve for x. In relationship to the 70 degree angle, the 13 unit side is opposite to it. Side x is the hypotenuse. Because of this, you use the sine function. Sine of an angle = length of opposite side/length of hypotenuse (sin(x) = O/H). You already know two of these values, you just need to solve for the third.
Now, I plug 70 degrees in for c and 13 in for the length of the opposite side. I plug in x for length of hypotenuse.
sin(70) = 13/x I need to isolate x on one side x(sin(70))=13 x=sin(70)/13 x = 13.834