I assume you have a right triangle.
To find the answer you need to say which of the three sides measures 7. Then we need to know which side is labelled x.
We have a choice of 3 sides for the side of length 7. That leaves 2 sides that could be labelled x.
There are three ratios you can use: sine, cosine, tangent.
The sides involved in these ratios are respectively: opposite÷hypotenuse, adjacent÷hypotenuse, opposite÷adjacent, where opposite means the side facing the angle 64 and adjacent the side on which the angle sits (trapped between this side and the hypotenuse).
Let’s assume for example that the hypotenuse is length 7, and x is opposite the angle 64°, then x/7=sin(64°), so x=7sin(64°).
Another example: 7 is opposite the angle and x is adjacent to the angle, then:
7/x=tan(64°), and x=7/tan(64°) (sometimes written 7cot(64°) or 7tan(90-64)=7tan(26°)).
Use tables or a calculator to find the sine, tangent, or whatever you need.
7sin(64°)=7×0.899=6.29 approx.
7/tan(64°)=7÷2.05=3.41 approx, or:
7tan(26°)=7×0.488=3.41.