Question

can someone explain how these equal 5 and -5 please

Answer 1

I'm not sure how you obtained -5 as your answer but this is the mathematical explanation. hope this helps!
${x}^{2} + 3x = 40 \\ {x}^{2} + 3x - 40 = 0 \\(x + 8)(x - 5) = 0 \\ x = - 8 \\ x = 5$