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\frac{7x^{2}}{21}+\frac{3\times 5x}{21}-\frac{8}{5}
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of 3 and 7 is 21. Multiply \frac{x^{2}}{3} times \frac{7}{7}. Multiply \frac{5x}{7} times \frac{3}{3}.
\frac{7x^{2}+3\times 5x}{21}-\frac{8}{5}
Since \frac{7x^{2}}{21} and \frac{3\times 5x}{21} have the same denominator, add them by adding their numerators.
\frac{7x^{2}+15x}{21}-\frac{8}{5}
Do the multiplications in 7x^{2}+3\times 5x.
\frac{5\left(7x^{2}+15x\right)}{105}-\frac{8\times 21}{105}
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of 21 and 5 is 105. Multiply \frac{7x^{2}+15x}{21} times \frac{5}{5}. Multiply \frac{8}{5} times \frac{21}{21}.
\frac{5\left(7x^{2}+15x\right)-8\times 21}{105}
Since \frac{5\left(7x^{2}+15x\right)}{105} and \frac{8\times 21}{105} have the same denominator, subtract them by subtracting their numerators.
\frac{35x^{2}+75x-168}{105}
Do the multiplications in 5\left(7x^{2}+15x\right)-8\times 21.