Given:
The base is a rectangle of dimension 7 cm×6 cm.
The length of each diagonal side = 10 cm
To find the height of MT.
Formula
By Pythagoras theorem we get,

where, h be the hypotenuse
b be the base and
l be the height.
Now, in this diagram,
The perpendicular distance of M from BC = 3 cm
Slant height = 10 cm
Taking, b = 3 and h = 10 we get,

or, 
or, 
or, 
Hence,
The height of MT is 9.5 cm.
For the answer tot he question above, let the number of t-shirts are represented by 't'
number of sweaters are represented by 's'
cost of 1t=10 and cost of 1s=20 so
10t+20s=2000 or (factor out the 10 if you want to simplify)
10(t+2s)=10(200)
divide both sides by 2
t+2s=200