If you look at the rectangle properly, you can see that the diagonal creates two triangles, and if we only focus on one triangle, we can see that the bad of the triangle is 7 and the perpendicular height is 4
Using Pythagorean theorem we can figure out the hypotenuse, or the diagonal
C squared is equal to a squared plus b squared
7^2+4^2 is 65
Square root of 65 is 8.1