In addition to solving equation \( ax+b=cx+d \) algebraically, you can also solve it graphically. When the lines \( y=ax+b \) and \( y=cx+d \) are graphed, you look for the intersection of these lines. In the pictures below are graphed lines \( y=ax+b \) and \( y=cx+d\). Choose the picture in which the equation \( ax+b=cx+d \) has only one non-negative solution.
A diagonal of a rectangle has length of \( 30\,\mathrm{cm} \) and the perimeter of the rectangle is \( 84\,\mathrm{cm} \). Find the difference of the length and the width of the rectangle in centimeters.
Two resistors of unknown resistances \( R_1 \) and \( R_2 \), where \( R_1 < R_2 \) are connected in series (Figure A) and total resistance of the circuit is \( R_S=64\,\Omega \). If the resistors are connected in parallel (Figure B), the total resistance \( R_P=15\,\Omega \). Find \( R_1 \).