$$x^{2}+2x+75-90=0$$
Subtract $90$ from $75$ to get $-15$.$$x^{2}+2x-15=0$$
To solve the equation, factor $x^{2}+2x-15$ using formula $x^{2}+\left(a+b\right)x+ab=\left(x+a\right)\left(x+b\right)$. To find $a$ and $b$, set up a system to be solved.$$a+b=2$$ $$ab=-15$$
Since $ab$ is negative, $a$ and $b$ have the opposite signs. Since $a+b$ is positive, the positive number has greater absolute value than the negative. List all such integer pairs that give product $-15$.$$-1,15$$ $$-3,5$$
Calculate the sum for each pair.$$-1+15=14$$ $$-3+5=2$$
The solution is the pair that gives sum $2$.$$a=-3$$ $$b=5$$
Rewrite factored expression $\left(x+a\right)\left(x+b\right)$ using the obtained values.$$\left(x-3\right)\left(x+5\right)$$
To find equation solutions, solve $x-3=0$ and $x+5=0$.$$x=3$$ $$x=-5$$