From the two right triangles, you can write the following equations using the Pythagorean theorem. Let's call that shared leg in the middle "y"
y^2 + b^2 = a^2
y^2 + c^2 = x^2
y^2 + b^2 = a^2
re-write this to get "y" alone for substitution.
y^2 = a^2 - b^2
substitute (a^2 - b^2) for y^2 in the other equation. y^2 + c^2 = x^2
a^2 - b^2 + c^2 = x^2
Now put in the values given for a,b,c to solve for x
(7.1)^2 - (5.6)^2 + (5.7)^2 = x^2
51.54 = x^2
square root
7.2 = x