Question 237373
The distance formula between two points is: 
{{{d=sqrt((x2-x1)^2+(y2-y1)^2)}}}
Now all you need to do is substitute values and solve algebraically.
{{{5=sqrt((4-1)^2+(a-1)^2)}}}
If you raise both sides of the eqn by the power of 2, you will get:
{{{25=(4-1)^2+(a-1)^2}}}
{{{25=3^2+(a-1)^2}}}
{{{25=9+(a-1)^2}}}
{{{16=(a-1)^2}}}
Now take the square root of both sides, you get:
{{{sqrt(16)=(a-1)}}}
{{{4+1=a}}}
{{{a=5}}}