Answer
Base =8
Altitude= 6
hypotenuse 10
Work Step by Step
By Pythagoras theorem $Hypotenuse^{2}= Base^{2}+ Altitude^{2}$
$(2x)^{2} = (x+3)^{2}+(x+1)^{2}$
$4x^{2} = x^{2}+9+6x+x^{2}+1+2x$
$2x^{2}-8x-10=0$
$x^{2}-4x-5=0$
$x^{2}-(5-1)x-5=0$
$x^{2}-5x+x-5=0$
$(x-5)(x+1)=0$
so x=5 is only valid value of x
Base= (x+3)=5+3=8
Altitude=(x+1)=5+1=6
hypotenuse=2x=10