The figure shows a right triangle at each corner with legs 3 and b and hypotenuse 4. The Pythagorean theorem tells you the sum of the squares of the legs is equal to the square of the hypotenuse:
3² + b² = 4² . . . . . . put the numbers into the Pythagorean formula
9 + b² = 16 . . . . . . . evaluate the squares
b² = 16 - 9 = 7 . . . . subtract 9 to isolate the variable
b = √7 . . . . . . . . . . undo the square operation by taking the square root